Updates to this story
Although he will continue as chief exec and keep his foot in any major decisions the day to day running of the company will go to Tim Cook, COO, for now.
In an email written to his employees and obtained by the Huffington Post, Jobs wrote: "At my request, the board of directors has granted me a medical leave of absence so I can focus on my health. I will continue as CEO and be involved in major strategic decisions for the company.
"I have asked Tim Cook to be responsible for all of Apple's day to day operations. I have great confidence that Tim and the rest of the executive management team will do a terrific job executing the exciting plans we have in place for 2011.
"I love Apple so much and hope to be back as soon as I can. In the meantime, my family and I would deeply appreciate respect for our privacy."
In late 2008 to mid-2009 Mr Jobs took leave for six months for a liver transplant. He has been dogged by ill health before. No word on what's wrong just yet.