Matching a donor heart with a recipient of the same gender seems to boost the odds of survival.
Researchers followed about 18,000 heart transplant patients for ten years.
They found gender matching reduced the risk of dying within 30 days of surgery by 25 percent, and lowered the one-year death rate by 20 percent.
Men who received a female heart had a 20 percent higher death rate than men who got a male heart.
Researchers say heart size seems to be the most logical explanation.
Experts say they are not recommending patients on a heart transplant list wait for a same-gender donor.
The risks of heart failure while waiting are far higher than the risk of organ rejection or death.