A random sample of almost 4,000 people nationwide found that where one of a pair of identical twins was homosexual there was a one in three chance that the second twin would also be gay.
Lead researcher Dr Kenneth Kendler from the Medical College of Virginia said: "By no means is sexual orientation genetically determined but clearly genes are playing some role by interacting with a range of environmental factors."
Dr Kendler examined data from 50,000 households collected by the MacArthur Foundation Midlife Development which included 794 sets of twins and 2,907 pairs of siblings.
Overall, 2.8 per cent reported that they were either homosexual or bisexual.
Among the 324 pairs of identical twins, 19 pairs reported one non-heterosexual member, while six sets said both were bisexual or homosexual.
This "concordance" rate was 31.6% among the identical twins, compared with 13.3% among non-identical twins of the same sex and 8.3% for all non-identical twins.
The researchers, writing in the American Journal of Psychiatry, said that "genetic factors may provide an important influence on sexual orientation".
The authors acknowledged the small numbers of twins but said it was the first study to look at the national probability of an inherited element to sexual orientation in this way.
"I was intrigued by the results," said Dr Kendler.
"It made sense to me that we were finding something that was moderately inheritable."
Concern
But Stonewall, the UK gay rights organisation has expressed concern over the purpose of the research.
Spokesman Sebastian Sandys said: "We're always suspicious when people start looking at the causes of homosexuality because when they do that they are usually looking for a cure.
"What matters is that there are lesbian and gay men with the same rights as everyone else, not trying to find out what makes them lesbian and gay."