The surprise announcement -- made on a U.S. market holiday -- dragged Apple shares down more than 6 percent in European trading. They are up 62 percent in the past 12 months on the Nasdaq stock exchange.
"Steve Jobs is seen by the market to be a major force in Apple's strategic direction," said Richard Windsor, global technology specialist at Nomura. "If his pancreatic cancer has returned, one could be quite worried."
Jobs' latest leave comes nearly two years after he took a six-month break to undergo a liver transplant. He also took time off after pancreatic surgery in 2004.
Apple has not dwelt on Jobs' health, and Jobs himself asked for respect for his privacy in a memo to employees made public on Monday.
In Jobs' absence, it will be up to chief operating officer Tim Cook to decide how much to tell investors about the absent chief executive, and what Apple plans to do with its $50 billion-plus pile of cash and investments. Apple faces Jobs questions
