Analysts are split on how crucial Steve Jobs is to Apple's future
Shares in Apple fell more than 2% after the computer giant's chief executive Steve Jobs said was taking medical leave until the end of June.
The Apple boss made the announcement after stock markets had closed on Wednesday, saying his health issues were more complex than he had 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 stand in while Mr Jobs is away.
The firm's shares were down by 4.8% early on Thursday before clawing back some losses to close 2.3% down at $83.22.
This was in addition to a 7.1% slide in extended trading on Wednesday, to one-year lows.
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.
"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.
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
Since then, Apple has produced out a string of sleek gadgets, from the iMac and the iPod to a new line of aluminium-covered Macbooks and the iPhone.
Many investors fear that without him, Apple will not be able to sustain its growth.
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."
Apple's board said it was giving him "complete and unwavering support".