To sort out the question, a new trial is now under way at a number of research sites around the country. The trial, funded by Baxter Healthcare, will use a specific type of blood-forming stem cell to treat heart-failure patients for whom all other treatments have been unsuccessful. These patients have chest pain, shortness of breath, and, often, an inability to exercise due to chronic insufficient blood flow to the heart.

A specific type of blood-forming stem cell will be injected directly into the heart muscle, a method that Raval suspects will work better than infusing the cells, a method used in previous studies. "Sending stem cells directly into the muscle might offer better cell retention and a better chance for cells to exert therapeutic effect," he says.

Researchers think that this type of cell will boost growth of new blood vessels, thereby increasing blood flow to the heart. "With heart failure, we think there is loss of microvasculature [the smallest blood vessels]," says Douglas Losordo, director of the Cardiovascular Research InstituteatNorthwestern University's Feinberg School of Medicine, in Chicago, and head of the multicenter trial. "That's what we're trying to treat with the cells."

Mesenchymal stem cells--the other type of stem cell in bone marrow--are also under scrutiny. This weekend Joshua Hare, chief of cardiology at University of Miami Miller School of Medicine, will report initial results of a trial testing these cells in heart-attack patients. Preclinical research suggests that these cells induce growth of new cells in heart muscle. While Hare won't comment on the specific findings before his announcement, he says that results are exciting. He now aims to start a larger trial for heart-attack patients, as well as an additional small-scale trial to test the therapy in patients with heart failure.

Hare's trial is unique in that he uses cells isolated from another person and then grown in the lab. The cells come from Osiris Therapeutics, a stem-cell company based in Baltimore that is sponsoring the trial. These cells appear not to trigger an immune reaction in the recipient--a fear when it comes to cell-transplant therapies. Patients did not receive immunosuppressants during the tests.

While all the human trials to date have used adult stem cells, scientists are not giving up on the potential of embryonic stem cells. These cells are easier to grow and manipulate, potentially providing a more abundant and controlled source of cells for transplant. Scientists have already been able to push embryonic stem cells to develop into clusters of heart cells that can actually beat, and they are now testing different cell types for their healing power. In some ways, ongoing tests with adult cells will prepare scientists for the day when embryonically derived cells are ready for human testing. "When someone tells me that they have a cell type that can regenerate new muscle or improve function in some way," says Raval, "we'll be ready to deliver them."