"It's a treatable form of prostate cancer. It was found at an early stage," the 55-year-old Giuliani said, sounding upbeat and frequently smiling during a news conference.

He said a biopsy was performed Wednesday, 2 weeks after a screening test indicated the followup test was necessary. Treatment options, which include radiation and chemotherapy, have not yet been decided.

Asked how it would affect his run for the Senate, the second-term Republican mayor: "I have no idea. ... I hope that I'd be able to run, but the choice that I'm going to make is going to be based on the treatment that's going to give me the best chance to have a complete cure."

When pressed on whether he would continue his campaign he said: "I don't think it's fair to answer questions about the Senate race right now. ... Should I do it? Would I be able to do it the right way? I hope that's the case but I don't know."

Clinton, asked upstate today how the announcement might affect the race, said: "Like all New Yorkers, my prayers and best wishes are with the mayor for a full and speedy recovery and I hope everyone joins me in wishing him well as he undergoes the treatment that is required."

The mayor noted his father died of prostate cancer in 1981, before the latest screening tests.

"It brings up very painful memories, and I miss my father every day of my life," he said.

Cancer of the prostate, a gland involved in semen production, is the second most common type of cancer found in American men, behind skin cancer. A total of 179,300 were diagnosed with the disease in 1999.

Treatments include removal of the chesnut-sized gland and radiation. Doctors also often recommend "watchful waiting," or leaving the cancer untreated, in elderly men who are more likely to die first from other ailments.

"The bad news is that it's cancer," the mayor said. "The good news is that there are lots of possible options, and it's going to take a while to figure out which option is going to bring about a cure," he said.

A Giuliani campaign spokeswoman, Juleanna Glover Weiss, said he will make his scheduled campaign trips in the coming week, including an appearance before a women's group in Saratoga on Friday.

About three-quarters of men with prostate cancer have elevated levels of a protein called prostate-specific antigen -- PSA. Testing blood for PSA has been a standard screening method for older men since about 1990. Giuliani said he had that test 2 weeks ago.

However, other conditions, such as noncancerous prostate enlargement, also can make PSA levels shoot up, so doctors must perform biopsies to make sure cancer is truly the cause. For every biopsy that detects prostate cancer, two or three others are false alarms.

Treatment options include radiation and chemotherapy. "Some forms of treatment would require taking some time off," Giuliani acknowledged, but said he didn't expect it to be a significant amount of time.

The mayor smiled frequently during the news conference and joked with reporters. Asked whether he would be nicer, he replied, "No way."

Rep. Rick Lazio, who had considered challenging Giuliani for the Republican nomination for the Senate, issued a statement offering his prayers and support for the mayor, adding that health issues transcend politics.

Through his spokesman, Lazio declined to say whether he would enter the race if Giuliani dropped out. The New York Senate primary is in September.