In an interview with Fox News Channel's Neil Cavuto, the former Tennessee senator, 64, said a doctor conducting a physical in 2004 found a bump on his neck, which turned out to be non-Hodgkin's lymphoma. He said the disease is in remission with no illness or symptoms, and it won't affect his decision about whether to seek the Republican nomination.
"I wouldn't be doing this if I wasn't satisfied in my own mind as to the nature of it and the fact that not only will I have an average lifespan but in the meantime I will not be affected in anyway by it," Thompson said. "Now of course nobody knows the future but that has been in the history for almost three years now in terms of no symptoms and no sickness."'
Thompson's physician, Dr. Bruce Cheson, hematology chief at Georgetown University Hospital, said the prognosis is good.
"Some lymphomas are very aggressive, but people with slow-growing types, like Sen. Thompson's, more often die from natural causes associated with old age, rather than from the disease," Cheson said in a statement.
Thompson's disclosure comes just weeks after Elizabeth Edwards, wife of Democratic presidential candidate John Edwards, revealed that her cancer had returned. White House spokesman Tony Snow recently underwent surgery for cancer that had spread to his liver.
Cancer has touched past and present presidential candidates. John McCain carries scars after three episodes of melanoma, the most aggressive form of skin cancer. His 2008 Republican rival, Rudy Giuliani, had prostate cancer, which also afflicted 2004 Democratic nominee John Kerry and 1996 Republican nominee Bob Dole.
In 1992, Democratic primary candidate Paul Tsongas was the first presidential candidate to run as a cancer survivor, having undergone a radical bone-marrow transplant six years earlier after lymphoma forced an end to his Senate career. Tsongas later died from a complication related to the treatment for the disease's recurrence.
"I have friends in politics, some in Congress, some running for president, and others who have successfully dealt with cancer," Thompson told interviewer Cavuto, who has battled Hodgkin's disease in the past and has multiple sclerosis. "It is certainly no respecter of persons and totally nonpartisan."
Lymphoma is an immune-system cancer that strikes more than 71,000 Americans a year, and kills more than 19,000 of them. The vast majority have the non-Hodgkin's form of lymphoma, a term that encompasses more than 30 different subtypes of the disease.
Some of these subtypes are termed "indolent," meaning they typically respond well to treatment - patients often go into remission for long periods, but the disease is not cured and may need to be battled back again periodically.
Survival varies widely, depending on the subtype of lymphoma. But overall, five-year survival for the non-Hodgkin's group of lymphomas is 63 percent, according to the American Cancer Society.
Thompson's doctor said he has a subtype known as "marginal zone lymphoma" and was treated with a drug called Rituxan, a monoclonal antibody which zeros in on cancer cells to specifically target them. But Cheson confirmed that Thompson is in remission with no signs of the cancer detectable by medical tests.
The former senator is an actor who has played many roles, including president of the United States, director of the CIA, an admiral and currently, district attorney Arthur Branch on NBC's long-running drama "Law & Order."
Thompson has said he would consider running for president in 2008, depending on how the rest of the Republican field shakes out.
"I'm just going to wait and see what happens," Thompson told Fox last month. "I wanted to see how my colleagues who are on the campaign trail do now, what they say, what they emphasize, what they're addressing, and how successful they are in doing that, and whether or not they can carry the ball in next November."
Thompson's rise to prominence began three decades ago, when he served as minority counsel to the Senate Watergate Committee. He famously asked one the key questions of the proceedings: "Mr. Butterfield, are you aware of the installation of any listening devices in the Oval Office of the President?"
That led to the disclosure of the Watergate tapes.
Starting in the late 1980s, Thompson got roles in numerous movies, including "No Way Out," "The Hunt for Red October," "Cape Fear and "In the Line of Fire."
Thompson was elected to the Senate in 1994 to fill the seat of Al Gore, who had been elected vice president. He won re-election to the Senate easily in 1996, but his daughter died of a heart attack during that term and he announced a couple months later that he wouldn't seek re-election.