Steve Jobs was diagnosed with pancreatic cancer in 2004
Apple chief executive Steve Jobs is being treated for a "hormone imbalance" but is staying on as the firm's head.
In a statement, Mr Jobs said he had been suffering from ill health for a number of months and had 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," said Mr Jobs.
Apple's board said it was giving him "complete and unwavering support".
Mr Jobs, who was diagnosed with pancreatic cancer in 2004, has appeared increasingly gaunt at public appearances, sparking rumours about his health.
Speculation has intensified since December, after Apple said Mr Jobs would not be making 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.
"The remedy for this nutritional problem is relatively simple and straightforward, and I've already begun the treatment.
"But, just like I didn't lose this much weight and body mass in a week or a month, my doctors expect it will take me until late this spring to regain it," he said.
"I will continue as Apple's CEO during my recovery."
In a written statement, Apple's board of directors said they were "lucky" to have Mr Jobs as its leader.
"As we have said before, if there ever comes a day when Steve wants to retire or for other reasons cannot continue to fulfill his duties as Apple's CEO, you will know it.
"He [Jobs] deserves our complete and unwavering support during his recuperation. He most certainly has that from Apple and its board."
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.