Ted Forstmann, the founding partner of private equity firm Forstmann Little and Co., which bought 24 Hour Fitness in 2005, is being treated for brain cancer.

Forstmann, 71, had surgery to remove a malignant brain tumor and is continuing medical treatments for the disease, according to multiple media reports, which surfaced Tuesday.

In a statement, Carl Liebert, CEO of San Ramon, CA-based 24 Hour, said Forstmann is still going to work every day at his office in New York.

“Ted Forstmann is currently dealing with some serious personal health issues that have recently arisen,” Liebert said. “The details of those issues are a private matter. It is business as usual at 24 Hour Fitness. All of us at 24 Hour Fitness are keeping Ted in our thoughts and prayers and hoping for his speedy recovery.”

Forstmann Little and Co. acquired 24 Hour from founder Mark Mastrov for $1.6 billion. After Forstmann made the offer to buy 24 Hour, he and Mastrov were in negotiations for more than two years before the deal was announced in 2005.

“24 Hour Fitness has a very strong market position, a superb management team and significant potential for growth, all characteristics we look for in an acquisition,” Forstmann said of the 24 Hour purchase in 2005. “We believe our unique capital structure can accelerate the growth of 24 Hour Fitness’ very impressive business model.”

24 Hour produced $1.352 billion in revenue in 2009, the most of any club company in Club Industry’s 2010 Top 100 Clubs list.

Mastrov stayed at 24 Hour and became chairman of the board when Liebert was hired as CEO. Mastrov left the company in 2008 and went on to become co-founder of New Evolution Ventures, Lafayette, CA, which owns Crunch, UFC Gym and Madonna’s Hard Candy Fitness.

“[Forstmann] is one of the last great corporate raiders/buyers,” Mastrov said in a statement. “I wish him a speedy and full recovery.”

Forstmann, whose net worth is reportedly $1.6 billion, also is chairman and CEO of IMG Worldwide, a sports and entertainment conglomerate that represents golfer Tiger Woods, tennis players Roger Federer and Rafael Nadal, and model Heidi Klum.

CNBC’s Darren Rovell reports that IMG CFO Mike Dolan will assume duties as the company’s president and COO due to Forstmann’s diagnosis. Forstmann will continue to serve as IMG’s chairman and CEO, Rovell adds.