Mr Allen battled another form of cancer more than 20 years ago
Paul Allen, co-founder of Microsoft, has been diagnosed with non-Hodgkin's lymphoma, his sister has said.
Jody Allen, the chief executive of Mr Allen's investment firm Vulcan Inc, told staff in a memo that her brother had begun chemotherapy.
Mr Allen, 56, founded Microsoft in 1975 with Bill Gates but left in 1983, when he was diagnosed with another form of cancer - Hodgkin's disease.
He survived that bout after receiving successful treatment.
"He received the diagnosis early this month and has begun chemotherapy," Jody Allen wrote to staff.
"Doctors say he has diffuse large B-cell lymphoma, a relatively common form of lymphoma.
"Paul is feeling OK and remains upbeat. He continues to work and he has no plans to change his role at Vulcan."
She added that he was "optimistic he can beat this".
After leaving Microsoft, Mr Allen founded Vulcan in 1986 to invest in media and communications companies.
He has also used his wealth to buy several sports teams.
He bought the Portland Trail Blazers NBA team in 1988, the Seattle Seahawks NFL team in 1997 and is a part-owner of the Seattle Sounders FC, a major league soccer team.
He also co-founded the Experience Music Project, a museum about rock music in Seattle.
He was named the 32nd richest person in the world, with a net worth of $10.5bn, in Forbes magazine's latest "rich list" of world billionaires.