Apple chief executive Steve Jobs is taking medical leave from the technology company for the second time in two years.
Mr Jobs said in a email to employees that he would continue as CEO and "be involved in major strategic decisions for the company."
He said he had asked Tim Cook, the firm's chief operating officer, to be responsible for day to day operations in his absence.
The company's shares fell almost 10% on the news.
Mr Jobs went on medical leave in 2009, and later disclosed he had undergone a liver transplant as part of treatment for pancreatic cancer.
Mr Jobs, who co-founded the company, is widely credited for its success, having revived its fortunes in 1996 after a 12-year absence from the company.
Apple launched the iPhone in 2007 and the iPad in 2010, and is believed to be preparing to launch a second version of its iPad.
The company has given no details of Mr Job's condition nor specified a time when he may return.