They also said they had no explanation for the higher complication rate in patients who knew they were being prayed for, in comparison to patients who only knew it was possible prayers were being said for them.

The work, which followed about 1,800 patients at six medical centers, was financed by the Templeton Foundation, which supports research into science and religion. It will appear in the American Heart Journal.

Dr. Herbert Benson of Harvard Medical School and other scientists tested the effect of having three Christian groups pray for particular patients, starting the night before surgery and continuing for two weeks. The volunteers prayed for "a successful surgery with a quick, healthy recovery and no complications" for specific patients, for whom they were given the first name and first initial of the last name.

The patients, meanwhile, were split into three groups of about 600 apiece: those who knew they were being prayed for, those who were prayed for but only knew it was a possibility, and those who weren't prayed for but were told it was a possibility.

The researchers did not ask patients or their families and friends to alter any plans they had for prayer, saying such a step would have been unethical and impractical.

The study looked for any complications within 30 days of the surgery. Results showed no effect of prayer on complication-free recovery. But 59 percent of the patients who knew they were being prayed for developed a complication, versus 52 percent of those who were told it was just a possibility.

Dr. Harold G. Koenig, director of the Center for Spirituality, Theology and Health at the Duke University Medical Center, who did not take part in the study, said the results did not surprise him.

"There are no scientific grounds to expect a result and there are no real theological grounds to expect a result either," he said.