Paul Allen, who co-founded Microsoft with his childhood friend Bill Gates, has died aged 65, his company Vulcan Inc. said in a statement on Monday.
Earlier this month Allen said the cancer he was treated for in 2009, non-Hodgkin's lymphoma, had returned.
Allen, who was an avid sports fan, owned the Portland Trail Blazers and the Seattle Seahawks.
He also founded the Allen Institute for Brain Science and aerospace company Stratolaunch.
Allen and Gates founded Microsoft Corp. in 1975. Microsoft's big break came in 1980, when IBM Corp. decided to move into personal computers. IBM asked Microsoft to provide the operating system.
The decision thrust Microsoft onto the throne of technology and the two Seattle-natives became billionaires. Both later dedicated themselves to philanthropy.
Allen served as Microsoft's executive vice president of research and new product development until 1983, when he resigned after being diagnosed with Hodgkin's disease.
Over the course several decades, Allen gave more than two billion US dollars to a wide range of interests, including ocean health, homelessness and advancing scientific research.