This report summarizes evidence that sexual orientation is familial. Family studies report that homosexual subjects have more homosexual siblings than do heterosexual subjects and more than would be expected given population frequencies. Twin studies find in general a higher concordance in sexual orientation among monozygotic than among dizygotic twins. Evidence for associated psychopathology in homosexual subjects or their relatives is inconclusive. Possible sources of bias in these reports are discussed and a suggestion is offered that family studies should be undertaken in which the sexual orientation of relatives would be directly ascertained by interview.