BOSTON, NOV. 11 -- Finnish researchers released a study today saying that raising the levels of so-called "good cholesterol" in the blood helps prevent heart attacks.

The Helsinki Heart Study, a five-year survey involving 4,081 otherwise healthy middle-aged men with high blood cholesterol levels, found a drug that lowers "bad cholesterol" and raises "good cholesterol" reduced the risk of heart attacks by at least 34 percent and risk of fatal heart attacks by 26 percent.

The study, published in the New England Journal of Medicine, offers "conclusive evidence" confirming previous findings that lowering "bad cholesterol" prevents heart disease, and the first direct evidence that raising "good cholesterol" offers even more protection, the researchers said.

The findings should provide added incentive for people to diet, exercise and quit smoking to try to lower "bad cholesterol" levels and raise "good cholesterol" levels, the Finnish researchers and other experts said.

If those efforts fail, the study also shows that the drug used in the study -- gemfibrozil -- is an apparently safe and effective alternative for treating patients who cannot achieve optimum blood cholesterol levels on their own, they said.

Low-density lipoprotein (LDL) is known as bad cholesterol because it can accumulate inside arteries. Previous studies have shown that lowering LDL levels could reduce the risk for heart disease.

High-density lipoprotein (HDL) is known as good cholesterol because it is believed to help remove fats from the blood. Studies have found that high HDL levels appeared to reduce risk of heart disease, but none had shown directly that increasing HDL levels could protect against heart disease.

In a separate discovery called "dramatic" by Nobel prize-winners, a team of American scientists reported that they may have found the mechanism by which cholesterol plaque builds up in the arteries.

Scientists from Genentech Inc. of San Francisco and the University of Chicago said they discovered that a single cholesterol-associated protein is remarkably similar to another protein involved in blood clotting.

"These dramatic findings may provide the long-sought link between lipoproteins and the clotting system," said Nobel laureates Michael Brown and Joseph Goldstein in a commentary in the British science journal Nature, which published results of the study.

About 40 million Americans are believed to have high blood cholesterol levels. Cholesterol accumulations inside blood vessels leading to the heart set the stage for heart attacks, the nation's leading killer.

In the Finnish study, 2,051 men were given gemfibrozil while 2,030 men received a a placebo, or inactive substance, twice a day for five years. The drug decreased LDL levels an average of 8 percent and increased HDL levels an average of 10 percent.

The overall rate of fatal and nonfatal heart attacks was 34 percent lower among the gemfibrozil group. When the researchers looked at the last three years of the study alone, the drug reduced the risk by almost 50 percent, indicating the benefits took a while to take effect, a Finnish official said.

The drug caused no significant adverse side effects, although the study indicated it might increase the incidence of gallstones.