Scientists measured the fingers of 18 pairs of female identical twins, where one was straight and the other gay.
On average, the lesbians, but not the straight twins, had different sized index and ring fingers, typically a male trait, but only on the left hand.
This may be the result of exposure to more testosterone in the womb, the University of Essex researchers said.
The scientists also measured the fingers of 14 pairs of male identical twins, where one was straight and the other gay, but found no link.
Both men and women were exposed to the "male" hormone, testosterone, in the womb - but some may be exposed more than others, the scientists said.
Study author Dr Tuesday Watts, from the psychology department at Essex University, said: "Because identical twins, who share 100% of their genes, can differ in their sexual orientations, factors other than genetics must account for the differences.
"Research suggests that our sexuality is determined in the womb and is dependent on the amount of male hormone we are exposed to or the way our individual bodies react to that hormone, with those exposed to higher levels of testosterone being more likely to be bisexual or homosexual.
"Because of the link between hormone levels and difference in finger lengths, looking at someone's hands could provide a clue to their sexuality."
The findings are published in Archives of Sexual Behaviour.
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