Early exposure to sex steroids is thought to be important in mediating the differentiation of male-typical sexual orientation. Bone morphology is a marker of childhood sex steroid exposure, because estrogens and androgens control sexual dimorphism in skeletal size. Anthropometric analysis of heterosexuals and homosexuals indicates that those bones, which become sexually dimorphic in childhood, but not those which become sexually dimorphic after puberty, are different in length in homosexuals and heterosexuals. Persons with a sexual preference for males have less long bone growth in the arms, legs and hands, than those with sexual preference for females. The data support the hypothesis that male homosexuals have had less steroid exposure during development than male heterosexuals and that female homosexuals have had greater steroid exposure during development than their heterosexual counterparts.