Apple share prices dipped 7% in late trading in the US after chief executive Steve Jobs announced he is to take a medical leave of absence until the end of June.
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Jobs told staff in an e-mail that his health problems were more complex than he had originally thought and chief operating officer Tim Cook will fill in for him while he is away.
Apple share prices have been edging down for several months in response to speculation about why Jobs has been losing weight and looking gaunt.
The announcement comes just a week after Jobs sought to allay fears by saying he was being treated for a relatively minor hormone imbalance and would be staying on as CEO.
"I have been losing weight throughout 2008. Doctors think they have found the cause - a hormone imbalance that has been robbing me of the proteins my body needs to be healthy," he said.
Last December, when the company revealed Jobs would not be at the 2009 Macworld show to deliver his traditional keynote, it said the decision had nothing to do with health concerns.
The firm said Jobs' absence was because the company would no longer be taking part in the long-running conference after this year.
Jobs was treated for pancreatic cancer in 2004, but he is not currently receiving cancer treatment according to several US media reports that cite sources close to the Apple boss.