A study at the Mayo Clinic in Minnesota has found that
prayer makes no
difference to patients' recovery rates; at least, if the patients don't
know about it. Half of the 799 heart disease patients in the study were
prayed for without their knowledge, and the other half were not, and
researchers found no difference between the two groups' recovery rates.
(Of course, if the patients knew they were being prayed for, it might be
different. And it's interesting to speculate on what the result would be
if half of the patients were told they were being prayed for, but
in fact, none (or all) were.)