Aspirin appears to significantly reduce the risk of colon cancer among people prone to the malignancy, the second-leading cause of cancer death in the United States, researchers reported yesterday.

Two highly anticipated studies found that taking aspirin every day cut the chances of developing growths that frequently turn into tumors among people who are predisposed to the growths or previously had colon cancer.

The findings provide the most definitive evidence yet that the common painkiller can reduce the risk for colon cancer, a notion that has been gaining credence in recent years based on strongly suggestive but indirect research. The results from one of the new studies were so striking that researchers terminated the project early because they decided it would be unethical to continue giving half the participants placebos.

Experts said the findings are likely to prompt many of the millions of people at risk for colon cancer to consider taking aspirin regularly. Millions of people already take an aspirin every day to reduce their risk of heart attack. Colon cancer strikes an estimated 147,500 Americans each year and kills 57,100.

"These studies are very important for the field of cancer prevention," said Ernest T. Hawk of the National Cancer Institute. "I would imagine there are a lot of patients that will choose to take an aspirin to try to decrease their risk."

Hawk and other experts cautioned that aspirin can carry dangers, most notably increasing the risk for bleeding, which can in rare cases cause strokes. The experts urged people to consult their doctors before taking aspirin regularly, and to continue to undergo regular exams to screen for colon cancer.

"Aspirin is not a magic bullet," said John A. Baron of Dartmouth Medical School, who led one of the new studies. "Nobody can take an aspirin and think that everything's fine and there is no need to see a doctor, or get screened, or take care of themselves in other ways."

But the research offers fresh and compelling substantiation for the emerging theory that drugs that suppress inflammation, such as aspirin, may reduce the risk for a variety of malignancies, experts said.

"This really establishes proof of principle," said Thomas F. Imperiale, a professor of medicine at the Indiana School of Medicine, whose critique of the new research also appeared in today's New England Journal of Medicine. "In that sense, it's a landmark."

Other over-the-counter painkillers that contain anti-inflammatory compounds, such as Advil and Motrin, may have similar effects, but that has not yet been demonstrated. The newer "superaspirins," such as Vioxx and Celebrex, can dampen the inflammatory response and are being studied for potentially larger beneficial effects. Tylenol is not an anti-inflammatory.

Inflammation is the earliest, most primitive response by the body's immune system to infection or injury. Although it causes redness and swelling when an injury occurs, it is crucial to survival.

But researchers have long known that inflammation can cause illnesses known as autoimmune diseases, such as lupus, Crohn's disease and rheumatoid arthritis. In recent years, evidence has mounted that inflammation also may play a role in more common diseases, including heart disease and cancer.

Aspirin has been shown to decrease the risk that heart attack survivors will suffer another attack. It may work by thinning the blood, but also by countering inflammation inside the walls of arteries supplying blood to the heart.

In cancer, the theory is that inflammation's ability to prompt cells to grow to heal wounds can go out of control, causing tumors to form.

Although there is evidence that inflammation plays a role in a variety of cancers, the strongest case has been for colon cancer. Previous studies found that people who were taking aspirin for other reasons, such as to reduce their risk for heart disease or alleviate arthritis pain, were less likely to develop colon cancer.

But the new studies represent the first time researchers have conducted well-designed studies aimed at testing that idea.

In the first study, Robert S. Sandler of the University of North Carolina in Chapel Hill and colleagues followed 517 patients who had survived a bout of colon cancer. Half were given 325 milligrams of aspirin a day, a standard dose, and the other half received a placebo.

The study was halted after about a year when researchers realized the volunteers taking aspirin were much less likely to develop growths called adenomas, a type of polyp prone to turning into colon cancer. Seventeen percent of those taking aspirin developed one or more adenoma, compared with 27 percent of those taking placebos.

In the second study, Baron and colleagues studied 1,121 patients with a history of adenomas, giving one-third 81 milligrams of aspirin, one-third 325 milligrams of aspirin and the last third placebos. Those receiving the low-dose aspirin were significantly less likely to develop new adenomas. For reasons researchers could not explain, the other group receiving aspirin showed no gain.