The mysterious part is, the patients didn't know anyone was praying for them.
The study's lead author says the simplest conclusion is that God answers prayer.
The study appears in today's issue of the Chicago-based American Medical Association's Archives of Internal Medicine.
Nearly 2,000 patients over a year's time admitted to the Mid America Heart Institute coronary care unit were randomly divided into two groups.
Those prayed for daily suffered about 10 percent fewer complications, ranging from chest pain to cardiac arrest.
Others say there's no hard evidence that praying makes a difference.