Microsoft co-founder Paul Allen, 56, one of the richest men in the world, is suffering from non-Hodgkin's lymphoma, according to his sister Jody Allen, the CEO of Vulcan Inc, his private asset management company.

Allen left Microsoft in 1983 to undergo radiation treatment and bone marrow transplant for Hodgkin's disease and, although successful, he never returned to Microsoft.

His sister sent this memo to Vulcan employees:

"I want to let you know that Paul was recently diagnosed with non-Hodgkin's lymphoma.

"He received the diagnosis early this month and has begun chemotherapy. Doctors say he has diffuse large B-cell lymphoma, a relatively common form of lymphoma.

"This is tough news for Paul and the family. But for those who know Paul's story, you know he beat Hodgkin's a little more than 25 years ago and he 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. His health comes first, though, and we'll be sure that nothing intrudes on that.

"We would ask you to respect Paul's privacy and not discuss this outside of the office.

"If you have any questions, please ask your EC member.

"Thank you in advance for what I know will be all your good thoughts for Paul."