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

In a memo sent to employees, Jody Allen, Paul Allen's sister and the CEO of his investment firm Vulcan Inc., said the 56-year-old received the diagnosis early this month. According to the memo, Paul Allen 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. The CEO wrote that 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."

Allen founded Microsoft with Bill Gates, a high school friend and fellow computer enthusiast, in 1975.

Gates said in a statement late Monday that Allen remains one of his closest friends.

"Melinda and I have Paul and his family in our thoughts and prayers," he said in the statement, as reported by TechFlash, a tech news Web site. "I know to him be a strong and resilient individual."

Allen served as the company's executive vice president of research and new product development until 1983, when he left to focus on his health.

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.

Beyond technology, Allen has used his Microsoft earnings to take his interests to an extreme. A longtime sports fan, Allen bought football's Seattle Seahawks and basketball's Portland Trailblazers, and he is part owner of the Seattle Sounders FC, a major league soccer team. Allen has been present at Seahawks games this season, chatting in the locker room with players. He has a band and a recording studio, and built the Experience Music Project, a museum about rock music in Seattle.

Allen has also collected and restored more than 30 vintage airplanes, started a brain science institute and through Vulcan's real estate arm redeveloped a large swath of downtown Seattle known as the South Lake Union neighborhood.

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