Microsoft co-founder Paul Allen said Monday that his non-Hodgkin's lymphoma has returned.

Allen was treated for the disease in 2009, and had been in remission. In a new statement, he said his doctors are treating it again, and he plans "on fighting this aggressively."

"A lot has happened in medicine since I overcame this disease in 2009," Allen said. "My doctors are optimistic that I will see good results from the latest therapies, as am I."

The Seattle billionaire said he will keep working on various projects, including his nonprofit research institutes and Vulcan, his investment firm.

Allen will also continue to be involved with the two professional sports teams he owns: the NFL's Seattle Seahawks and the NBA's Portland Trail Blazers.

Allen founded Microsoft (MSFT) with Bill Gates in 1975 and left in 1983, when he was treated for Hodgkin's lymphoma.

Non-Hodgkin's lymphoma, like the less-common Hodgkin's disease, is a cancer of the lymphatic system.