"I have asked [COO] 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," Jobs said in the letter.
Jobs did not reveal his current illness, and he asked for his employees to respect his privacy.
During the first leave in 2004 he was treated for a rare form of pancreatic cancer; during second period he underwent a liver transplant. In both cases Jobs' illness was not revealed until after he had recovered.
Apple will release its quarterly earnings report Tuesday afternoon.