Older men with early-stage prostate cancer are not taking a big risk if they keep an eye on the disease instead of treating it right away, suggests the largest study to look at this issue since PSA tests became popular.

Only 10 percent of the 9,000 men in the study who chose to delay or skip treatment had died of prostate cancer a decade later. The vast majority were alive without significantly worsening symptoms or had died of other causes.

Even the 30 percent who eventually sought treatment were able to delay it for an average of 11 years.

"It is important news," said Dr. Otis Brawley, chief medical officer of the American Cancer Society. "It may persuade some middle-of-the-roaders that we are overtreating this disease," and that PSA testing might be amplifying the problem, he said. The PSA blood test to help detect tumors has been widely used since the 1990s.

Grace Lu-Yao of Robert Wood Johnson Medical School in New Jersey led the study and will report results at a cancer conference later this week in San Francisco.

Whether to treat prostate cancer is one of the biggest medical dilemmas today. The disease is the most common cancer in American men — about 220,000 cases will be diagnosed this year — but most tumors grow so slowly they never threaten lives. There is no sure way to tell which tumors will.

PSA tests can help find tumors many years before they cause symptoms, but routine screening of men at average risk of the disease is not recommended, because there is no proof it saves lives.

Prostate cancer treatments are tough, especially on older men. Many men are left with sexual or bladder control problems. Some doctors instead recommend "watchful waiting" to monitor signs of the disease and treat only if they worsen, but smaller studies have given conflicting views of the safety of that approach.

The new study looked at the natural course of the disease in men who chose that option. It is the first involving so many older men — half were over 75 — and so many whose tumors were found through PSA tests.

Using the federal government's cancer database, researchers studied 9,018 men diagnosed from 1992 to 2002 with early-stage prostate cancer who did not get surgery, radiation or hormone therapy for at least six months. Most never got any treatment at all.

A decade later, 3 percent to 7 percent of those with low- or moderate-grade tumors (rated by how aggressive the cells appear) had died of prostate cancer, versus 23 percent of those with high-grade tumors. Overall, prostate cancer killed 10 percent of them.

The National Cancer Institute paid for the study. It is not the final word — that usually comes from studies where similar groups of patients are randomly assigned to get one treatment or another, and the results compared.