A study of more than 1,800 patients who underwent heart bypass surgery has failed to show that prayers specially organized for their recovery had any impact, researchers said Thursday.

In fact, the study found some of the patients who knew they were being prayed for did worse than others who were only told they might be prayed for -- though those who did the study said they could not explain why.

The patients in the study at six U.S. hospitals included 604 who were actually prayed for after being told they might or might not be; another 597 patients who were not prayed for after being told they might or might not be; and a group of 601 who were prayed for and told they would be the subject of such prayer.

The praying was done by members of three Christian groups in monasteries and elsewhere -- two Catholic and one Protestant -- who were given written prayers and the first name and initial of the last name of the prayer subjects. The prayers started on the eve of or day of surgery and lasted for two weeks.

Among the first group -- who were prayed for but only told they might be -- 52 percent had post-surgical complications compared to 51 percent in the second group, the ones who were not prayed for though told they might be. In the third group, who knew they were being prayed for, 59 percent had complications.

After 30 days, however, the death rates and incidence of major complications was about the same across all three groups, said the study published in the American Heart Journal.

"Our study was never intended to address the existence of God or the presence or absence of intelligent design in the universe" or to compare the efficacy of one prayer form over another, said the Rev. Dean Marek, director of chaplain services at the Mayo Clinic, one of the authors.