Thousands of people diagnosed with severe heart disease could soon be given fresh hope.
Thousands die of heart failure each year
Doctors in the United States say they have successfully used stem cells to treat patients with heart failure - a condition where the heart is unable to pump enough blood around the body.
Their results add to growing evidence the technique could help patients to live longer.
The procedure involves taking stem cells from a patient's bone marrow and injecting it directly into the heart muscle.
Stem cells are master cells, which have the capability of turning into any other cell. In this case, scientists believe they turn into new blood vessel or heart muscle cells.
Doctors at the Texas Heart Institute and the Federal University in Rio de Janeiro tested the technique on 21 Brazilian patients.
The patients all had very severe heart disease.
"These patients were desperately ill," said Dr James Willerson who led the trial.
He said: "They had a relatively high risk of dying and had no other forms of therapy available because their heart failure was so severe."
This procedure could lead to an effective treatment for severe heart failure
Doctors injected 14 of the 21 patients' hearts with stem cells obtained from their own bone marrow.
Each patient received an average of 15 injections, containing about two million stem cells. The remaining seven patients did not have the treatment.
After two months, patients who underwent treatment had significantly less heart failure and angina.
Their hearts were also able to pump more blood around their body. The improvements were still visible four months after treatment.
However, two patients did die. A patient who did not undergo treatment died two weeks after the start of the trial. A second patient who did undergo treatment died 14 weeks later.
Nevertheless doctors hailed the results. Dr Willerson said they would have to be replicated in larger clinical trials before doctors elsewhere could carry out the procedure.
He said: "This is one of the largest series of stem cell treated patients reported so far.
"If our findings are confirmed in larger trials, this procedure could lead to an effective treatment for severe heart failure."
He added: "We studied a small number of patients and we will need to study a much larger number in many medical centres to identify the benefit and risks that might exist."
The study is published in the American Heart Association journal Circulation.