Portland Trail Blazers and Seattle Seahawks owner Paul Allen said Monday the non-Hodgkin’s lymphoma for which he was treated in 2009 has returned.

Allen, the Microsoft (Nasdaq: MSFT) co-founder who's since made a name for himself in the fields of science and geologic exploration, said he has already begun his treatment regimen.

"I plan on fighting this aggressively," he said in a release. "A lot has happened in medicine since I overcame this disease in 2009. My doctors are optimistic that I will see good results from the latest therapies, as am I."

Allen said he'll continue to stay active in his Vulcan group of companies, as well as the Allen Institutes, the Seahawks and Trail Blazers.

"I have confidence in the leadership teams to manage their ongoing operations during my treatment," he said.

Allen promoted Blazers president Chris McGowan to lead the Vulcan sports management side just last week.

The Blazers start play Oct. 18. The Seahawks have started the season by splitting their first four games.

"I am very grateful for the support I’ve received from my family and friends," Allen said. "And I’ve appreciated the support of everyone on the teams and in the broader community in the past, and count on that support now as I fight this challenge."