Paul Allen, a private investor and co-founder of Microsoft, has been told he has non-Hodgkin’s lymphoma, according to an e-mail message sent to employees of his investment firm, Vulcan.

This month, Mr. Allen, 56, received the diagnosis of diffuse large B-cell lymphoma, a relatively common form of lymphoma, according to the message, sent by Jody Allen, Vulcan’s chief executive and Mr. Allen’s sister.

Mr. Allen received a diagnosis of Hodgkin’s disease in 1983, and left Microsoft for radiation therapy and a bone marrow transplant. He never returned to the company’s day-to-day operations, but his stock in the rapidly growing software firm made him the 17th richest man in the United States, according to Forbes magazine’s annual list.

“This is tough news for Paul and the family,” wrote Ms. Allen. “Paul is feeling O.K. 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.”