New research, presented Wednesday at the American Heart Association’s Scientific Sessions 2008, reveals that heart transplant patients have better chances of survival and a lower risk of rejection if they get organs from donors of the same sex.

It is not the first time researchers find such an association. Previous studies have shown hints that gender differences may play a role in other types of transplants, such as lungs or kidneys. However, this is the largest study by far to find an effect.

The study was led by Dr. Eric Weiss, a cardiac surgery researcher at Johns Hopkins University in Baltimore and was paid for by the federal government.

The researchers found that men who received hearts from smaller women had the worst results. Therefore, they concluded that the pumping capacity of the organ is crucial to the success of the procedure. When it comes to women, they were also somewhat more likely to reject transplants from males, perhaps because of lingering immune stimulation from earlier pregnancies. On the other hand, same-sex transplant appeared to have better outcomes.

“We generally don’t assume that organs from males and female donors have inherent differences affecting long-term outcomes, but our data suggest that there are important differences which must be taken into account. Heart size would seem to be the most important factor, beyond that, no-one knows why sex matching is important to transplant survival,” Dr. Weiss said.

He and his colleagues studied records belonging to the United Network for Organ Sharing (UNOS) for 18,240 heart transplants that took place between 1998 and 2007. Some patients were followed for as long as 10 years. About 71 percent of the patients, 77 percent of males and 51 percent of females, in the study received a heart from a donor of the same sex.

About a quarter of the transplant patients died during the study. However, the researchers found that when the sexes of the donor and recipient matched, there was a 13 percent lower risk of graft rejection in the first year, a 14 percent lower risk of rejection over the entire length of the study , a 25 percent drop in death during the first 30 days after transplant and a 20 percent lower rate of death in the first year.

“What was interesting was the substantial difference in the long term, as well,” Dr. Weiss said.

Unfortunately, for many patients things don’t look too optimistic. According to UNOS, about 2,700 Americans are waiting for a heart, and only 2,200 heart transplants are done each year – some of them second operations for people whose first transplant failed. Researchers said it is preferable to have a heart from a person of the opposite-sex rather than to die waiting for a same-sex transplant.

“You're still much better off getting a heart transplant" than waiting and risking your own heart will give out before another becomes available,” Dr. Weiss said.