Doctors testing a new treatment for heart attacks said today they have restored life to seemingly dead heart muscles by seeding them with cells borrowed from patients' thigh muscles or bones.

The idea is to find an alternative to transplants for people whose hearts are so damaged that they fail to pump blood forcefully enough. This condition, called heart failure, is a growing health problem that afflicts an estimated 5 million people in the United States alone.

Two years ago, a French doctor described a novel alternative: He put millions of immature skeletal muscle cells into the badly damaged heart of a 72-year-old man. His heart began to pump more powerfully, although it was unclear whether the improvement came from the new cells or from coronary bypass surgery he received at the same time.

The physician, Philippe Manasche of Bichat Hospital in Paris, has repeated the approach on 10 patients. Similar experiments are being conducted by teams in the United States, Germany, England and Poland.

Preliminary but encouraging data on these experiments were reported today at the annual scientific meeting in Chicago of the American Heart Association. Doctors said the shifted cells can live inside the heart's dead scar tissue and show at least some signs of contracting as the original heart muscle would.

"This is quite exciting and definitely new," said Timothy Gardner of the University of Pennsylvania, who is not involved in the studies.

For now, all researchers can say for sure is that the transferred cells take root and flourish in dead areas of the heart. Whether they make the heart pump more forcefully remains to be seen, although researchers say they see encouraging evidence this may happen.

"The results so far support the hypothesis that these cells will do some good. It gives us a reason to go on," said Francis Pagani of the University of Michigan.

Pagani is working with Nabil Dib of the Arizona Heart Institute, whose team tested the approach on 16 patients getting coronary bypasses or temporary pumps to keep them alive until they could have heart transplants.

Ordinarily, the heart pushes out more than half of its blood with each beat. Dib's patients had such bad heart failure that their hearts pumped just 23 percent. After the bypasses and cell injections, this improved to 36 percent, although it was impossible to say how much, if any, of the new strength resulted from the extra cells.

Like Manasche, Dib's team begins with immature muscle cells, called myoblasts, obtained from the patients' own thighs. These are grown in test tubes until millions are available. Then they are injected into parts of the heart that died during heart attacks.

"We clearly showed living tissue in the injected scar," Dib said. "If this proves efficacious, we will improve the quality of life of our patients and their survival. This will replace heart transplants."

Tomasz Siminiak of the University School of Medical Science in Poznan, Poland, tested the same approach on 10 patients. Improved contraction was seen in scarred areas of the heart within a month of the procedure.

Both Manasche and Siminiak found that patients needed drugs to prevent potentially lethal heart rhythm disturbances in the months after the injections, although this hazard appeared to go away with time.

Manuel Galinanes of the University of Leicester in Britain and Christof Stamm of Rostock University in Germany are using primitive bone marrow cells instead of thigh muscle, and both say their results are similar.