Paul Allen, who co-founded software company Microsoft, has died age 65

Microsoft Corp co-founder Paul Allen, who had been suffering from non-Hodgkin's lymphoma, a form of cancer, has died at the age of 65, his family says.

Allen had said earlier this month that he had started treatment for non-Hodgkin's lymphoma, the same type of cancer he overcame nine years ago.

Born in Seattle, Allen attended Washington State University for two years before dropping out to co-found Microsoft in 1975 with Bill Gates, a friend of his from high school.

Allen, who left Microsoft in 1983, set up investment firm Vulcan Inc in 1986 to manage his multibillion-dollar portfolio.

"Paul Allen's contributions to our company, our industry and to our community are indispensable," Microsoft Chief Executive Officer Satya Nadella said in a statement.

"As co-founder of Microsoft, in his own quiet and persistent way, he created magical products, experiences and institutions, and in doing so, he changed the world."

Allen, who ranked 44th on Forbes magazine's 2018 list of billionaires with a $US20.5 billion ($A28.7 billion) fortune, was a major benefactor in Seattle, funding everything from libraries and universities to brain research.

His investments include ownership stakes in several professional sports teams, including the NBA's Portland Trail Blazers, NFL's Seattle Seahawks and Seattle Sounders FC of Major League Soccer.