NEW YORK — Does praying for a sick person's recovery do any good?

In the largest scientific test of its kind, heart surgery patients showed no benefit when strangers prayed for their recovery.

And patients who knew they were being prayed for had a slightly higher rate of complications. Doctors could only guess why.

Several scientists questioned the concept of the study.

Science "is not designed to study the supernatural," said Dr. Harold G. Koenig, director of the Center for Spirituality, Theology and Health at the Duke University Medical Center.

The researchers emphasized that their $2.4 million study could not address whether God exists or answers prayers made on another's behalf. The study could look only for an effect from the specific prayers offered as part of the research, they said.

The study "did not move us forward or backward" in understanding the effects of prayer, said Dr. Charles Bethea, a co-author and cardiologist at the Integris Baptist Medical Center in Oklahoma City. "Intercessory prayer under our restricted format had a neutral effect."

Dr. Herbert Benson of Harvard Medical School, co-principal investigator of the study, agreed. "We cannot come to a conclusion, except to say that by this study design, with its limitations, this is what we found," he said.

Researchers also said they didn't know why patients who knew they were being prayed for had a higher rate of complications than patients who only knew that such prayers were a possibility.

Maybe they became anxious by the knowledge that they'd been selected for prayers, Bethea said: "Did the patients think, 'I am so sick that they had to call in the prayer team?' "

The study only focused on prayers by strangers, they said.

It's the largest and best-designed study ever to test the medical effects of intercessory prayers — praying on behalf of someone else.

But critics said the question of God's reaction to prayer simply can't be explored by scientific study.

The study followed about 1,800 patients at six medical centers. It was financed by the Templeton Foundation, which supports research into science and religion, and one of the participating hospitals. It will appear in Tuesday's issue of the American Heart Journal.

The research team 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 — their identities known only by 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 didn't 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.

Koenig, of Duke University Medical Center, who didn't take part in the study, said the results didn't 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.

"There is no god in either the Christian, Jewish or Muslim scriptures that can be constrained to the point that they can be predicted."

Within the Christian tradition, God would be expected to be concerned with a person's eternal salvation, he said, and "why would God change his plans for a particular person just because they're in a research study?"