Doctors say coronary bypass surgery should be reserved as a last resort because a 12-year study shows the benefits of the popular operation diminish after five years.

Bypass surgery, in which a shunt is built around the blocked section of artery, is the most common open-chest heart operation. Nearly 300,000 Americans a year undergo the operation, which has become a $6 billion-a-year industry in the United States.The new findings published in Thursday's New England Journal of Medicine basically support two other large bypass-surgery studies conducted in the 1970s.

In addition, a recent report in the Journal of American Medical Association found that nearly half of the patients who underwent bypass surgery in three hospitals should not have had the operations or could have done without them.

The new study was reported by European researchers and was based on observation of 767 men who had stable angina, intermittent cramping chest pain caused by lack of oxygen to the heart. The condition can be caused by artery blockage and is considered a possible precursor to heart attack or sudden death.

Researchers led by Dr. Edvardas Varnauskas of Sahlgrenska Hospital in Goteborg, Sweden, treated the men either with surgery or drugs.

"The decrease in the survival rate after five years was more pronounced in the surgically treated than in the medically treated group," the scientists wrote.

At the five-year mark, 92 percent of those who had bypass surgery were still alive compared with an 83 percent survival rate for those treated with drugs. Survival declined significantly to the near-equal point over the seven subsequent years, until it was 71 percent for surgery patients and 67 percent for those who had taken drugs.