The e-mail was sent by Paul Allen's sister Jody Allen, who is chief executive officer of Vulcan Inc., Paul Allen's investment firm.
Non-Hodgkin's lymphoma is a cancer of the lymphatic system. The e-mail said Allen has "diffuse large B-cell lymphoma," The Wall Street Journal reported Tuesday.
The e-mail also says Allen is "optimistic he can beat" the disease. Twenty-five years ago, Allen won a battle with Hodgkin's disease. That diagnosis was made before he left Microsoft, the Journal said.
"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," the e-mail from his sister reads.
In a statement, Microsoft's other co-founder Bill Gates said, "Paul is among my closest friends, and I know him to be a strong and resilient individual."