Apple chief executive officer Steve Jobs is being treated for a "hormone imbalance" but is staying on as the firm's CEO.
Apple now admits that Jobs will not be addressing tomorrow's Macworld show in San Francisco because of the illness.
Last December, when the company revealed he would not be at the 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 run by IDG after this year.
But in a statement, Jobs said he had been suffering from ill-health for a number of months.
"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.
Job said he would carry on as CEO, but added that his treatment for his hormone imbalance would not be completed until April, when he hopes to have gained weight. His increasingly gaunt appearance was a cause of concern throughout last year.