Steve Jobs has suffered dramatic weight loss in the past year
Apple chief executive Steve Jobs is to take medical leave until the end of June, saying his health issues were more complex than he first thought.
Mr Jobs revealed last week he was being treated for a "hormone imbalance" but was staying on as the firm's head.
Chief operating officer Tim Cook will again stand in while Mr Jobs is away.
Diagnosed with pancreatic cancer in 2004, Mr Jobs had appeared increasingly gaunt at recent public appearances, sparking rumours about his health.
Speculation about his health had intensified since December, after Apple said Mr Jobs would not be making his annual keynote address at the Macworld conference in San Francisco.
In an e-mail to employees on Wednesday, Mr Jobs said: "Unfortunately the curiosity over my personal health continues to be a distraction not only for me and my family, but everyone else at Apple as well."
"In order to take myself out of the limelight and focus on my health... I have decided to take a medical leave of absence until the end of June."
"As CEO, I plan to remain involved in major strategic decisions while I am out. Our board of directors fully supports this plan."
In last week's statement, Mr Jobs said doctors thought they had found the cause for his weight loss: "A hormone imbalance that has been robbing me of the proteins my body needs to be healthy."
Apple's board said it was giving him "complete and unwavering support".
Apple's share price fell 7% in late trading on Wall Street following the announcement, showing how closely the company's fortunes are still identified with the person who helped create it, says the BBC's Greg Wood in New York.
Professor Gerard Tellis of Marshall Business School at University of Southern California, said Mr Jobs had a "sense of what the market really wants and that's a rare talent".
"In the case of iPod, iTunes, the iPhone, as well as the iMac, he seems to have got it right."
"In each case, the company made a big decision based on his tastes and they seem to have got it right, because those products have turned out very well," he said.
Apple's shares have risen and fallen over the last year in step with rumours or news about Mr Jobs' health.
While the top executive's health would be an issue for investors in any company, at Apple the level of concern has been heightened, because Mr Jobs is involved in everything from ideas for new products to the way they are marketed.
Adopted and raised by Clara and Paul Jobs
Dropped out of Reed College in Portland at 17 after one term
A keen Beatles fan, has said his business model was based on the group
Best known for wearing a black turtleneck, Levi's jeans, and sneakers
"Steve Jobs is the Ronald McDonald of Apple, he is the face," said Rob Enderle of the Enderle Group in Silicon Valley.
"They either need to redefine the company so his role is divided among different people or they need to find somebody that can clone Steve Jobs."
But other analysts disagreed.
"Everyone is going to speculate he is on his deathbed, like it usually goes. The company will do just fine with Steve taking a leave of absence," said Van Baker at information technology research firm Gartner.
"Apple is not going to collapse without him there."
Mr Jobs co-founded Apple with Steve Wozniak in 1976 at the beginning of the personal computer revolution. He left the firm in 1985 but returned in 1997 and became full-time chief executive once again in 2000.
Since then, Apple has churned out a string of sleek gadgets, from the iMac and the iPod to a new line of aluminum-covered Macbooks and the iPhone.
Many investors fear that without him, Apple would not be able to sustain its growth.
Mr Jobs said he was passing day-to-day management to Tim Cook, who filled in for Mr Jobs in 2004 when the Apple chief took leave to battle his cancer.