And patients who knew they were being prayed for had a higher rate of post-operative complications like abnormal heart rhythms, perhaps because of the expectations the prayers created, the researchers suggested.
Because it is the most scientifically rigorous investigation of whether prayer can heal illness, the study, begun almost a decade ago and involving more than 1,800 patients, has for years been the subject of speculation.
The question has been a contentious one among researchers. Proponents have argued that prayer is perhaps the most deeply human response to disease, and that it may relieve suffering by some mechanism that is not yet understood. Skeptics have contended that studying prayer is a waste of money and that it presupposes supernatural intervention, putting it by definition beyond the reach of science.
At least 10 studies of the effects of prayer have been carried out in the last six years, with mixed results. The new study was intended to overcome flaws in the earlier investigations. The report was scheduled to appear in The American Heart Journal next week, but the journal's publisher released it online yesterday.
In a hurriedly convened news conference, the study's authors, led by Dr. Herbert Benson, a cardiologist and director of the Mind/Body Medical Institute near Boston, said that the findings were not the last word on the effects of so-called intercessory prayer. But the results, they said, raised questions about how and whether patients should be told that prayers were being offered for them.
"One conclusion from this is that the role of awareness of prayer should be studied further," said Dr. Charles Bethea, a cardiologist at Integris Baptist Medical Center in Oklahoma City and a co-author of the study.
Other experts said the study underscored the question of whether prayer was an appropriate subject for scientific study.
"The problem with studying religion scientifically is that you do violence to the phenomenon by reducing it to basic elements that can be quantified, and that makes for bad science and bad religion," said Dr. Richard Sloan, a professor of behavioral medicine at Columbia and author of a forthcoming book, "Blind Faith: The Unholy Alliance of Religion and Medicine."
The study cost $2.4 million, and most of the money came from the John Templeton Foundation, which supports research into spirituality. The government has spent more than $2.3 million on prayer research since 2000.
Dean Marek, a chaplain at the Mayo Clinic in Rochester, Minn., and a co-author of the report, said the study said nothing about the power of personal prayer or about prayers for family members and friends.
Working in a large medical center like Mayo, Mr. Marek said, "You hear tons of stories about the power of prayer, and I don't doubt them."
In the study, the researchers monitored 1,802 patients at six hospitals who received coronary bypass surgery, in which doctors reroute circulation around a clogged vein or artery.
The patients were broken into three groups. Two were prayed for; the third was not. Half the patients who received the prayers were told that they were being prayed for; half were told that they might or might not receive prayers.
The researchers asked the members of three congregations — St. Paul's Monastery in St. Paul; the Community of Teresian Carmelites in Worcester, Mass.; and Silent Unity, a Missouri prayer ministry near Kansas City — to deliver the prayers, using the patients' first names and the first initials of their last names.
The congregations were told that they could pray in their own ways, but they were instructed to include the phrase, "for a successful surgery with a quick, healthy recovery and no complications."
Analyzing complications in the 30 days after the operations, the researchers found no differences between those patients who were prayed for and those who were not.
In another of the study's findings, a significantly higher number of the patients who knew that they were being prayed for — 59 percent — suffered complications, compared with 51 percent of those who were uncertain. The authors left open the possibility that this was a chance finding. But they said that being aware of the strangers' prayers also may have caused some of the patients a kind of performance anxiety.
"It may have made them uncertain, wondering am I so sick they had to call in their prayer team?" Dr. Bethea said.
The study also found that more patients in the uninformed prayer group — 18 percent — suffered major complications, like heart attack or stroke, compared with 13 percent in the group that did not receive prayers. In their report, the researchers suggested that this finding might also be a result of chance.
One reason the study was so widely anticipated was that it was led by Dr. Benson, who in his work has emphasized the soothing power of personal prayer and meditation.
At least one earlier study found lower complication rates in patients who received intercessory prayers; others found no difference. A 1997 study at the University of New Mexico, involving 40 alcoholics in rehabilitation, found that the men and women who knew they were being prayed for actually fared worse.
The new study was rigorously designed to avoid problems like the ones that came up in the earlier studies. But experts said the study could not overcome perhaps the largest obstacle to prayer study: the unknown amount of prayer each person received from friends, families, and congregations around the world who pray daily for the sick and dying.
Bob Barth, the spiritual director of Silent Unity, the Missouri prayer ministry, said the findings would not affect the ministry's mission.
"A person of faith would say that this study is interesting," Mr. Barth said, "but we've been praying a long time and we've seen prayer work, we know it works, and the research on prayer and spirituality is just getting started."