The 13-month trial was the first to compare the safety and effectiveness of so-called mesenchymal, or bone marrow-derived, stem cells taken from patients themselves versus those provided by donors.
Such adult stem cells that renew themselves and mature into specific cell types have been used for 40 years in bone marrow transplants.
Scientists are now exploring their use as treatments for ailments such as heart disease and inflammatory conditions, some of the biggest markets in medicine.
The rationale behind using patients' own stem cells to treat disease is that they do not trigger an attack by the body's immune system. Mesenchymal stem cells, however, are also not recognized as foreign tissue.
Researchers from the University of Miami Miller School of Medicine, funded by the National Institutes of Health, found that previously prepared cells from a healthy donor were comparatively safe and may offer the most convenience since it takes up to eight weeks to grow the amount of stem cells needed for the treatment.
The study involved 30 patients whose hearts were damaged by an earlier heart attack. Half received heart-muscle injections of their own cells, while the other half received donor cells.
Scar tissue was reduced by 33 percent in both groups, a result researchers called "very, very significant."
Improvements in heart function were seen in 28 percent of those receiving donor cells, and in 50 percent of patients receiving their own cells.
After a year, five patients in the donor cell group and eight who received their own cells suffered serious adverse events.
"The trials so far have very small patient numbers," said Stefanie Dimmeler, director of the Institute of Cardiovascular Regeneration Center of Molecular Medicine at Johann Wolfgang Goethe University in Frankfurt, Germany. "I think this early work in cardiac stem cells look very promising."
The trial results were presented here at the annual scientific meeting of the American Heart Association and published in the Journal of the American Medical Association.
Companies working to develop off-the-shelf stem cell treatments include Celgene Corp, Pluristem Therapeutics Inc, Athersys Inc and Mesoblast Ltd.
(Editing by Bernard Orr)