Apple boss Steve Jobs has announced that he is to take "medical leave" from the company.
In an e-mail to employees he said he was taking the break to focus on his health.
He said he would continue as chief executive of Apple and be involved in any major decisions. Day-to-day running of the company will pass to Tim Cook.
In late 2008 to mid-2009 Mr Jobs was absent from Apple for six months to have a liver transplant.
It was part of the series of treatments he has undergone for pancreatic cancer. He was first diagnosed as suffering from the cancer in 2004 and underwent surgery later that year to remove a tumour from his pancreas.
In his e-mail Mr Jobs said he would be back at work as soon as he can.
"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," he said in an e-mail.
"I have asked Tim Cook to be responsible for all of Apple's day to day operations."
Mr Cook is currently the firm's chief operating officer. He has run the company day-to-day before now during previous times when Mr Jobs has been dealing with his health problems.
The announcement was made on a public holiday in the US when there is no trading in company stocks and shares.
However, Apple shares closed down 6.4% on the Frankfurt stock exchange. Year-on-year, they are up 79% and over 24 months up 328%.
The news comes ahead of Apple's first quarter results, due on 18 January.
Mr Jobs is an iconic presence at Apple and is widely credited as the architect of its current run of success based around products such as the iPad and iPhone.
The absence comes as Apple is rumoured to be preparing to launch the second version of its iPad - the successor to the tablet computer it launched in 2010.