For people facing surgery or fighting disease, the prayers of others can be a comfort. But a large U.S. study has found that praying for the health of a heart bypass patient has no impact on their surgical outcome.

The prayer of others is widely believed to influence the recovery of patients. To find out if it works, several studies have been carried out with mixed results. But researchers at six academic hospitals across the United States found that the investigations had not followed the best scientific methods. So they carried out their own study on 1,800 patients about to undergo heart bypass surgery, in which clogged arteries are replaced by clean ones removed from the leg. It is the largest known study so far on the question.

A total of 600 of the patients received the prayers of others before surgery after being told they might or might not get them. Another 600 were told the same thing, but were not prayed for. The third group of 600 received pre-surgical prayer and knew it.

The researchers report that the prayers had no effect on the recovery of the first two groups of heart patients, those who did not know whether others were praying for them. Co-investigator Herbert Benson, a physician at Harvard Medical School in Boston, says the effect of a patient's prayer for his or her own health differs. He says the patient may be more relaxed and that has been found beneficial to health.

"We were not studying the relaxation response in this study," said Mr. Bensen.  "We were studying the extension of that. Could external prayer perhaps do the same thing? In this particular study, we did not find that was the case."

To their surprise, the researchers found that the third group of heart bypass patients, those who knew others were praying for them, had more surgical complications as a whole than the other two. Another of the researchers, physician Charles Bethea of the Oklahoma Heart Institute, says the doctors do not know if that result is random or not. He says people in this group might have become anxious knowing others were praying for them.

"Did the patient think that, 'Am I so sick that they had to call in the prayer team?'" he asked.

The researchers are careful to note that they did not intend to address the existence of God or to compare the effectiveness of one type of prayer over another.

They concede that their research is not the final word on the matter of so-called distance prayer and say that it should stimulate further study. For one thing, co-investigator Jeffrey Dusek, a Harvard Medical School psychologist, points out that it involved prayer by strangers, not by family or close friends, so the sense of community that might have influenced a patient's sense of well-being was not present.

"A single study does not answer the question," he noted.  "I strongly suggest that that not be the case. I'm hoping that family and friends will continue doing what they have done for years and pray for whomever they want to and loved ones before surgery."

The study on prayer appears in the American Heart Journal.