Apple chief executive Steve Jobs is taking medical leave, two years after having a six-month break when he underwent a liver transplant.
Mr Jobs, a survivor of pancreatic cancer, said chief operating officer Tom Cook would take responsibility for day-to-day operations but he would continue to be chief executive and be involved in major strategic decisions.
"At my request, the board of directors has granted me a medical leave of absence so I can focus on my health," Mr Jobs, 55, wrote in an email to staff published on a regulatory newswire. "I love Apple so much and hope to be back as soon as I can."
Apple has been closely identified with the charismatic Mr Jobs, who revived the computer maker's fortunes in 1996 after a 12-year absence from the company he co-founded.
Analysts said the effect on Apple's operations should be limited in the short term, since its product line-up was strong, but his absence would be a worry if it became prolonged. Mr Cook ran day-to-day operations during Jobs's last absence in 2009.
"It wasn't expected. This will come as a surprise to Apple investors and definitely take some shine off the Apple stock," said Alexander Peterc, equity analyst at Exane.
"But even if Steve Jobs never returns to Apple, I would not expect a visible, tangible impact on how Apple is executing over the next couple of years."
Richard Windsor, global technology specialist at Nomura, agreed that Mr Jobs absence should not have a fundamental effect, but added: "Perception of the company is another matter.
"Steve Jobs is seen by the market to be a major force in Apple's strategic direction.
"If his pancreatic cancer has returned, one could be quite worried."
Apple is due to report quarterly results today and is expected to say that revenues rose 50 per cent. Its performance will have been boosted by sales of its iPad tablet computer.