Allen, who is a survivor of Hodgkin's disease, has been diagnosed with non-Hodgkin's lymphoma, according to Vulcan CEO Jody Allen, who is also Paul Allen's sister.
Fellow Microsoft co-founder Bill Gates said he and his wife, Melinda, were keeping Allen in their thoughts.
"Melinda and I have Paul and his family in our thoughts and prayers," Gates said in a statement. "Paul is among my closest friends, and I know to him be a strong and resilient individual."
For more, read "Paul Allen diagnosed with cancer" on CNET News.