Steven P. Jobs, the chief executive of Apple, at the company's headquarters in October.

Mr. Jobs wrote in a letter to Apple employees, released after markets closed, that he had learned over the past week that his health issues were “more complex” than he originally thought. He said he planned to return to Apple at the end of June and in the meantime would hand day-to-day control of Apple over to Timothy D. Cook, its longtime chief operating officer.

Mr. Jobs, 53, wrote that curiosity over his personal health “continues to be a distraction not only for me and my family, but everyone else at Apple as well.” He said he would maintain the chief executive title and stay involved in major strategic decisions.

Mr. Jobs offered no new details about the cause of his health problems. In a letter last week that was meant to calm fears about his condition, he called it a “hormonal imbalance” that was robbing his body of proteins and causing him to lose weight. Mr. Jobs recovered from pancreatic cancer after surgery in 2004 but has appeared unusually gaunt at recent appearances.

Two people who are familiar with Mr. Jobs’s current medical treatment said he was not suffering from a recurrence of cancer, but a condition that was preventing his body from absorbing food. Doctors have also advised him to cut down on stress, which may be making the problem worse, these people said.

An Apple spokesman, Steve Dowling, said the company had no further comment on the issue beyond Mr. Jobs’s letter.

Apple shares dropped sharply in after-hours trading following the release of the letter, losing $5.63, or 6.6 percent, to $79.70. The stock fell 2.71 percent in regular trading amid a broad market slump.

Charles Wolf, an analyst at Needham & Company who follows Apple, said the stock market would probably fear the worst.

“It is reasonable to expect, given the history of Steve’s illness, that the market is probably going to assume that he is not going to return to Apple,” Mr. Wolf said.

Mr. Jobs’s leave of absence is the latest twist in a saga that has left the company’s shareholders, analysts and ardent fans exasperated and straining to divine the hidden meanings in the company’s vaguely worded communications.

Last June, when Mr. Jobs appeared strikingly thin at a company conference for programmers, an Apple spokeswoman said he was recovering from a “common bug.” Soon afterward, Mr. Jobs acknowledged to The New York Times that he was suffering from digestive difficulties related to a surgical procedure he had as part of his cancer treatment.

Then last week, Mr. Jobs sought to calm speculation about his withdrawal from his regular keynote speech at the annual Macworld conference by acknowledging he had a “hormonal imbalance.”

“The letter last week pretty much tried to reassure people that his health condition was extremely minor, but obviously it is more serious than first thought,” said Ryan Jacob, founder of the Los Angeles-based Jacob Internet Fund, which owns a stake in Apple. “It’s disturbing.”

Some shareholders and analysts have expressed frustration with the trickle of news coming from Apple about Mr. Jobs’s health.

A spokesman for the Securities and Exchange Commission, John Nester, declined to comment on Apple’s situation. But he said that in general, while there were no specific requirements for companies to disclose the health of their officers or directors, companies needed to assess whether health issues could have a material impact on results.

For most companies, such information is not crucial because they are not so closely associated with one person. But Apple may be an exception. Since he co-founded Apple in 1976, and particularly since he returned to it in 1997 after a decade-long absence, Mr. Jobs has been inextricably linked to the company and its brand.

Over the last eight years, he has, seemingly single-handedly, powered Apple back to the forefront of the technology industry. Apple has sold 180 million iPod music players, and over the last year, it has sold more than 20 million units of its slender iPhone.

But Mr. Jobs does not run Apple alone, and now at least one of his deputies will get a moment in the sun. Mr. Cook joined Apple in 1998 from the computer maker Compaq and is responsible for the company’s manufacturing and sales operations.

“I don’t think we know enough about Tim since he has never really been in the limelight,” said Toni Sacconaghi, an analyst at Sanford C. Bernstein & Co. “What we can say is Apple has a complicated business model with enormous seasonality. But it has been exceptionally well run across a number of dimensions for a number of years. I think a lot of that credit goes to Tim.”

By all accounts, Mr. Cook does not have the long-term vision or showmanship of Mr. Jobs, who appears capable of peering around corners into the future of technology, and can whip crowds into a frenzy merely by taking something new out of his pocket.

That is why analysts and shareholders saw so much portent in Mr. Jobs’s 170-word letter.

“These are times where you reflect about what Steve Jobs means for the company,” said Gene Munster, an analyst at Piper Jaffray. “At the end of the day, investors need to come to grips with the reality of a post-Steve Jobs world. This is the most urgent wakeup call they have had.”

Philip W. Schiller, Apple's senior vice president of worldwide product marketing, at the Macworld Expo in San Francisco on Tuesday.

Beginning this week, three of the four major music labels — Sony Music Entertainment, Universal Music Group and Warner Music Group — will begin selling music through iTunes without digital rights management software, or D.R.M., which controls the copying and use of digital files. The fourth, EMI, was already doing so.

In return, Apple, whose dominance in online music sales gives it powerful leverage, agreed to a longstanding demand of the music labels and said it would move away from its insistence on pricing all individual song downloads on iTunes at 99 cents.

Instead, the majority of songs will drop to 69 cents beginning in April, while the biggest hits and newest songs will go for $1.29. Others that are moderately popular will remain at 99 cents.

The music companies are hoping that their eagerly awaited compromise with Apple will give a lift to digital downloads. They will be able to make more money on their best-selling songs and increase the appeal of older ones.

And with the copying restrictions removed, people will be able to freely shift the songs they buy on iTunes among computers, phones and other digital devices.

Technologically sophisticated fans of digital music complain that D.R.M. imposes unfair restrictions on what they can do with the tracks they have bought. For example, the protected files from iTunes do not work on portable players made by companies other than Apple.

“I think the writing was on the wall, both for Apple and the labels, that basically consumers were not going to put up with D.R.M. anymore,” said Tim Bajarin, an analyst with Creative Strategies, a market research company.

Music industry watchers widely applauded the move and said it could help digital music sales, which have shown signs of slowing down just five years after Apple introduced iTunes.

In particular, lower prices for some songs could spur consumers “to buy deeper into the catalog, and expand their relationship with digital music,” said Russ Crupnick, an analyst with the NPD Group.

The music industry could use a lift. Sales of CDs fell 20 percent last year from 2007. About 2.4 billion songs were bought on iTunes in the last year, aided by Apple’s expansion into international markets. But that was not nearly enough to make up for losses in traditional retail stores.

Industry pundits have long pointed to D.R.M. as one culprit for the music companies’ woes, saying it alienated some customers while doing little to slow piracy on file-sharing networks.

Apple has been campaigning against D.R.M. at least since February 2007, when the chief executive, Steven P. Jobs, wrote an open letter criticizing the software. Apple reached a deal with EMI that year to offer music without the copying restrictions.

But it could never reach the same agreement with EMI’s larger rivals. Sony, Warner and Universal allowed other online music services, like Amazon’s MP3 Store, to sell unprotected music, but they withheld it from Apple. Their goal, industry analysts say, was to try to strengthen online rivals to iTunes, which they viewed as having a dangerous level of control over their business.

“Apple definitely wanted to remove D.R.M. from music, but the record labels would not allow them to renegotiate their licensing agreements, because they wanted to help competitors succeed against Apple in the market,” said Bill Rosenblatt, president of GiantSteps Media Technology Strategies, a consulting firm.

Apple, for its part, appeared to resist variable pricing, fearing it would amount to a price increase for the most popular tracks on iTunes, which constitute the bulk of sales on the service. It has also said the consistent 99-cent price made things simpler for buyers.

It is not clear what broke the impasse, but the deteriorating economy may have put pressure on music companies.

“For the major labels, it was clearly time for them to accelerate becoming digital music companies in a macroeconomic environment that is downright frightening,” said Greg Scholl, chief executive of The Orchard, a digital distributor of music from independent labels.

The compromise gives the recording industry new leverage over their online music sales, Mr. Scholl added. They can start to sell new tracks at the higher price, then gradually drop prices to keep sales moving. Labels could also experiment with bundled packages of songs and even special editions at higher prices.

Harry Wang, director of mobile product research at the consulting company Parks Associates, said, “They aren’t going to get a huge amount of money from this new arrangement, but in an ailing music industry, anything that can provide more money will be better than the status quo.”

Apple said customers would be able to pay a one-time fee to strip copying restrictions from music they have already bought on iTunes, at 30 cents a song or 30 percent of the album price. ITunes customers can achieve the same effect by burning all of their music to a CD and then reimporting the music into the iTunes software, although this reduces sound quality somewhat.

The company also said that its popular iPhone would be able to download songs from iTunes over wireless data networks like AT&T’s. Previously, iPhone owners had to either attach the phone to a computer or connect to a local Wi-Fi network.

Apple reported the changes in iTunes at its keynote presentation at the annual Macworld conference in San Francisco, given by Philip W. Schiller, Apple’s senior vice president for worldwide marketing.

Mr. Jobs was not at the event, after disclosing this week that he had a treatable hormone problem that had resulted in significant weight loss over the last year.

Apple's chief executive Steven P. Jobs at a media event in September.

For the last ten years, attending Steven P. Jobs’s annual keynote address at the Macworld Expo in San Francisco has been a sacred pilgrimage for Apple enthusiasts. That ritual has now ended.

Apple announced on Tuesday that Mr. Jobs would not appear at the Macworld conference in January and that the company’s presentation would instead be delivered by Philip Schiller, Apple’s senior vice president of worldwide product marketing. Apple also said it would pull out of the conference after this year’s event.

The news unleashed a fresh wave of speculation among Apple fans about what this portends for the computer maker’s lineup of products — and also for Mr. Jobs’s health, which has been subjected to intense scrutiny over the last few years.

In a statement, Apple strove to undercut any downbeat interpretation of the news, explaining that it was abandoning Macworld because it was able to unveil new products at its own events, like its Worldwide Developers Conference, held in June in San Francisco, and what has become an annual event each September at its campus in Cupertino, Calif., devoted to the iPod.

The company is also now in a position to broadcast those events online and in its growing network of 250 well-trafficked stores around the world, reaching a broader audience than it could inside any single convention hall.

“Phil is giving the keynote because this will be Apple’s last year at the show,” said Steve Dowling, an Apple spokesman. “It doesn’t make sense for us to make a major investment in a trade show we will no longer be attending.”

Despite that explanation, the news that Mr. Jobs would not be making his anticipated public appearance at Macworld struck the predictable chords of anxiety among those who follow Apple. As bloggers and analysts traded rumors and theories about what it all means, Apple investors also reacted to the news, sending Apple shares down as much as 6 percent in after-hours trading.

Mr. Jobs is a survivor of pancreatic cancer and has appeared unusually thin at recent appearances, though Apple says his health is fine.

Another explanation is that Apple’s slate of new product announcements in January is so unexciting that it does not require the considerable presentation talents of Mr. Jobs himself. In past years Mr. Jobs used the Macworld stage to unveil the iPhone and the skinny MacBook Air.

The Apple rumor mill has predicted that for January’s show Apple is readying a new version of the Mac Mini, a boxy computer that is sold without a monitor, and a smaller version of the iPhone. If true, Neither announcement would induce paroxysms of delight in the Apple faithful.

“Maybe they do have some surprising news and they are giving Phil Schiller a chance to introduce some exciting products,” said Gene Munster, an analyst at Piper Jaffray. “But the likelihood of that is low.”

Apple has appeared in recent months to be making an effort to shine the public spotlight on executives other than Mr. Jobs. Timothy D. Cook, Apple’s chief operating officer, delivered a presentation to press and analysts at the October introduction of new Macintosh laptops.

Apple executives have also said they have a succession plan, but they have not publicly discussed it.

Tuesday’s announcement was a clear blow to IDG, a technology media company based in Framingham, Mass., that has been staging Macworld since 1984. This year’s conference has already been hit by the news that several large companies, including Belkin, a computer accessories maker, and Adobe, the software publisher, would be scaling back their presence at the show for economic reasons.

Paul Kent, an IDG vice president and the general manager of the Macworld show, said he expected roughly the same number of exhibitors and attendees this year as last year. He would not comment specifically about Apple’s news, but he affirmed IDG’s commitment to staging the show.

“We look forward to many successful years of Macworld to come,” Mr. Kent said, reading from a prepared statement.