Microsoft co-founder and billionaire investor Paul Allen was diagnosed with non-Hodgkin's lymphoma and is undergoing chemotherapy.

In a memo sent to employees, Jody Allen, Paul Allen's sister and the chief executive officer of his investment firm Vulcan Inc, said the 56-year-old received the diagnosis early this month.

According to the memo, Mr Allen has diffuse large B-cell lymphoma, a relatively common form of lymphoma.

Mr Allen battled another form of immune system cancer, Hodgkin's lymphoma, more than 20 years ago and survived. Jody Allen wrote that Mr Allen "is optimistic he can beat this, too.

"Paul is feeling OK and remains upbeat," she added. "He continues to work and he has no plans to change his role at Vulcan."

Mr Allen founded Microsoft with Bill 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.

Mr Allen remained a major shareholder and member of the board, and went on to invest broadly in technology, real estate, sports and the arts.

He formed Vulcan in the mid-1980s to invest in media and communications companies, including America Online, DreamWorks Animation and cable operator Charter Communications Inc.

He also co-founded a Silicon Valley research lab that he then shuttered after investing more than 100 million US dollars.

Beyond technology, Mr Allen has used his Microsoft earnings to take his interests to an extreme. A longtime sports fan, Mr Allen bought American football's Seattle Seahawks and basketball's Portland Trailblazers, and he is part owner of the Seattle Sounders FC, a major league soccer team.

At the last count, Mr Allen's net worth totalled about 11.5 billion US dollars, making him the 17th richest person in the world according to Forbes' September 2009 tally.

Belfast Telegraph