Paul Allen, who founded Microsoft with Bill Gates, has been diagnosed with cancer.
Allen, 56, has begun chemotherapy treatment for non-Hodgkin's lymphoma.
His sister and the CEO of his investment firm Vulcan Inc, Jody Allen, released a memo to his employees to say he had been diagnosed earlier this month.
The memo says that he has diffuse large B-cell lymphoma, a relatively common form of lymphoma.
Allen battled another form of immune system cancer, Hodgkin's lymphoma, more than 20 years ago and survived.
Allen's sister wrote that her brother "is optimistic he can beat this, too."
"Paul is feeling OK and remains upbeat. He continues to work and he has no plans to change his role at Vulcan," the Telegraph quoted her as saying.
Allen founded Microsoft with Gates, a high school friend and fellow computer enthusiast, in 1975.
He served as the company's executive Vice President of research and new product development until 1983, when he left to focus on his health.