British researchers reported yesterday that a study of 5,000 doctors who took an aspirin a day failed to show that the drug prevented heart attacks. But even as they published their study, the researchers said it should be discounted.

Rather, they said, more weight should be given to a much larger U.S. study that showed aspirin can reduce heart attacks. Results of the U.S. study were described in a widely publicized announcement this week.

The reason for the backhanded announcement was the relative size of the studies.

The U.S. study involved a carefully controlled group of 22,071 doctors. For five years, half the group took one tablet, 325 milligrams, every other day. The key result was that those had about half the heart attacks of those who did not take the aspirin.

Yesterday, a research group at Oxford University's Radcliffe Infirmary reported on a study of 5,000 doctors who took 500 milligrams of aspirin daily for six years. The key result was their study showed no difference in heart attacks between doctors who took aspirin and ones who did not.

The trouble was that the British study involved too few participants to demonstrate the effect of the larger study, said Dr. Charles H. Hennekens of Harvard Medical School. Hennekens was a leader of the U.S. study group and was also a coauthor of the British study.

The statistical power of one study was greater, he said.

In simple terms, he said, "We did a study here of 22,000 people over five years. The study there was of 5,000 people over six years. So we are talking about the difference between 120,000 person-years in one study versus 30,000 person-years in the other."

If the U.S. study had been stopped at the same point as the British study, when it had only a quarter of the information, Hennekens said, "We wouldn't have seen the effect, either. Our study would have been reported as a negative study."

In the British paper, the authors said their study showed no effect of aspirin but noted that the American study was several times as large, "so the positive result from the United States carries more weight than the null result from the United Kingdom."

One thing both showed is that a small number of "hemorrhagic strokes" may be caused by taking aspiring regularly.

The effect of aspirin which is believed to be responsible for both stopping heart attacks and causing strokes is that it helps prevent blood platelets from forming clots in the vessels.

Thus, blockages that might stop the flow to the heart muscle would not develop as easily. But in some cases bleeding might be encouraged; bleeding in the brain can become a hemorrhagic stroke.