The chance of long-term survival after a heart transplant rises if the person getting the new heart is the same sex as the donor, researchers say.
The US study of more than 18,000 operations found death rates rose by a fifth above average levels if it was a woman who had received a man's heart.
Same-sex transplants had lower rates of rejection over the next few years.
The precise reason is unclear - although the difference in size could be a factor.
"We generally don't assume that organs from male 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"
Men and women's hearts are anatomically identical, and with donor organs in short supply, many transplants will involve a heart taken from a donor of the opposite sex to the patient.
Scientists at Johns Hopkins University in Baltimore suggest that this may not be the best way to use them.
They looked back at heart transplants which happened between 1998 and 2007, comparing survival rates in matched against non-matched transplants.
Overall, seven out of ten operations involved matched transplants.
They found a 13% lower risk of rejection - where the body recognises the new heart as "foreign" and attacks it - in matched transplants over the first year.
There was a 25% lower rate of death within 30 days of the operation in the matched group.
Further analysis revealed that the worst scenario appeared to be a woman receiving a "male" heart, while the most successful operations were those involving male recipients and male donors.
Dr Eric Weiss, who led the study, said: "We generally don't assume that organs from male 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."
However, he said that if the patient was in severe heart failure, then receiving an non sex-matched organ would still be better than waiting longer.
Professor Robert Bonser, a cardiac surgeon from Queen Elizabeth Hospital in Birmingham, said that while the findings were "interesting", the scarcity of organs meant that gender matching was unlikely to become the rule.
He said: "In the UK there are too few donors to make gender matching really practical - the researchers recommend that patients do not wait longer for a same sex organ, and we would certainly agree with that."
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