Apple chief executive Steve Jobs is being treated for a "hormone imbalance" but is staying on as head of the company.
In a statement, Mr Jobs said he had been suffering from ill health for a number of months and lost weight throughout 2008.
He said: "Doctors think they have found the cause: 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".
Mr Jobs was diagnosed with pancreatic cancer in 2004. The BBC reports he has appeared increasingly gaunt at public appearances, sparking rumours about his health.
Speculation has intensified since December, after Apple said Mr Jobs would not make his annual keynote address at the Macworld conference in San Francisco.
In the statement, Mr Jobs said that doctors did not expect him to return to his normal weight until later this year.
Mr Jobs co-founded Apple in 1976, but left in 1985 to start another computer venture. He returned to Apple in 1997 and became full-time chief executive once again in 2000.