Animal studies have shown that prenatal exposure to the anticonvulsant drugs phenobarbital and phenytoin alters steroid hormone levels which consequently leads to disturbed sexual differentiation. In this study, possible sequelae of prenatal exposure to these anticonvulsants on gender development in humans were investigated. A follow-up study was carried out in phenobarbital- and phenytoin-exposed subjects and control subjects matched for age, sex, and the mothers' ages. Subjects were born in the Academic Medical Center between 1957 and 1972. Out of 243 exposed and 222 control subjects who were asked to volunteer, 147 exposed subjects (72 male, 75 female) and equal numbers of their matched control subjects participated in the follow-up study. They were interviewed and were asked to fill out questionnaires on gender role behavior, gender development, and sexual orientation. As a group, exposed and control subjects did not differ with respect to gender role behavior, although higher numbers of prenatally anticonvulsant-exposed subjects reported current or past cross-gender behavior and/or gender dysphoria. Three prenatally anticonvulsant-exposed subjects were transsexuals and had undergone sex reassignment surgery, a remarkably high rate given the rarity of transsexualism. In addition, two exposed males had exclusively homosexual experiences, whereas none of the control males reported exclusive homosexual behavior. The groups did not differ in attainment of pubertal psychosexual milestones.