CHICAGO -- Heart patients who had someone praying over them -- without their knowledge -- suffered 10 percent fewer complications, a study found.

"It's potentially a natural explanation we don't understand yet. It's potentially a super- or other-than-natural mechanism," said William S. Harris, a heart researcher who was lead author of the study.

Harris and other researchers at the Mid America Heart Institute, the heart program of St. Luke's Hospital in Kansas City, Mo., studied 990 patients admitted during a year to the institute's coronary care unit.

The patients were randomly divided into two groups. In one, patients were prayed for daily by community volunteers for four weeks; the other patients didn't have anyone assigned to pray for them.

The patients, their families and their caregivers were not even told they were in a study. The volunteers were told only the first names of the patients and asked to pray daily for their speedy recovery with no complications.

After four weeks, the prayed-for patients had suffered about 10 percent fewer complications, ranging from chest pain to cardiac arrest, researchers reported in Monday's issue of the Archives of Internal Medicine, published by the American Medical Association.

The findings suggest that prayer might be an effective addition to standard medical care, the researchers said.

At the same time, Harris acknowledged that his study had limitations. Among other things, many patients in the comparison group undoubtedly had friends and relatives praying for them, too.

An expert not involved in the work said the study and a similar 1988 one in San Francisco that involved 393 heart patients had questionable methods.

Both studies tallied complications using their own scoring systems, which have not been proved medically valid, said the expert, Dr. Herbert Benson, a professor of medicine at Harvard Medical School and president of the Mind/Body Medical Institute at Beth Israel Deaconess Medical Center in Boston.

Other studies have found no apparent benefits to being prayed for, and in one, prayed-for patients actually fared worse, Benson said.

Benson said medical research has shown that people who believe in God or in prayer generally fare better than those who don't. What remains unproven is whether prayer itself makes a difference, he said.