And Bolli says there was a significant improvement in the ability of the stem cell recipients to exercise.

The study participants also got MRI scans before and after treatment. A year after getting the stem cells, the images showed a 30 percent reduction in the scars of dead heart muscle.

"The reduction of the scar implies that new heart muscle is being produced to replace that scar. And so it is evidence that the stem cells are regenerating new heart muscle."

This ongoing study is very small, with just 16 patients in the stem cells group, plus seven more in the control group. But a commentary published with his paper says Bolli's findings "raise new optimism" because of the study's rigorous quality and unexpectedly good results.

Bolli published his findings in The Lancet and presented them at the American Heart Association meeting in Florida.

Also at that Heart Association meeting: another report of stem cell research, though with less promising results. In a study published in the journal of the American Medical Association, JAMA, researchers gave recent heart attack victims stem cells taken from their bone marrow. The idea was to prevent the onset of heart failure. Earlier studies had suggested the technique might work. But in this case, after six months there was no improvement in heart function.