BACKGROUND Prenatal exposure to diethylstilbestrol causes infertility in male mice and has been associated with malformations of the genital tract in men. However, little is known about the fertility of men who have been exposed prenatally to diethylstilbestrol. METHODS In 1950 through 1952, 1646 pregnant women were enrolled in a randomized, placebo-controlled clinical trial of diethylstilbestrol at Chicago Lying-in Hospital. We interviewed men who were born to the women during that study about their fertility. RESULTS Four decades after their birth, we were able to trace 548 of the surviving sons (68 percent). Ninety percent consented to be interviewed (253 who had been exposed to diethylstilbestrol in utero and 241 who had not been exposed). Congenital malformations of the genitalia were reported three times as often by the diethylstilbestrol-exposed men as by the sons of the women in the placebo group. Within the exposed group, malformations were reported twice as often among those exposed to diethylstilbestrol before the 11th week of gestation as among those exposed later (P = 0.05). Men with genital malformations were nonetheless as fertile as other men. The diethylstilbestrol-exposed men (with or without genital malformations) had no impairment of fertility by any measure, including whether they had ever impregnated a women, age at the birth of their first child, average number of children, medical diagnosis of a fertility problem, or length of time to conception in the most recent pregnancy of the female partner. Finally, diethylstilbestrol-exposed men had no impairment of sexual function, as indicated, for example, by the frequency of intercourse or reported episodes of decreased libido. CONCLUSIONS High doses of diethylstilbestrol did not lead to impairment of fertility or sexual function in adult men who had been exposed to the drug in utero.