The in utero effects of diethylstilbestrol on the human male genital tract are reported in our followup study of male offspring of mothers treated with diethylstilbestrol. Anatomical and functional abnormalities were significantly greater in male patients exposed to diethylstilbestrol compared to male controls whose mothers were all participants in a prospective, randomized double-blind study on the effects of diethylstilbestrol on pregnancy at our hospital during the early 1950s. Epididymal cysts, hypotrophic testes and capsular induration of the testes were among the more common genital lesions found in more than 25 per cent of 159 male patients exposed to diethylstilbestrol compared to a 6.8 per cent incidence in 161 male controls. Spermatozoal analysis revealed severe pathological changes (Eliasson score more than 10) in 32 per cent of 31 patients exposed to diethylstilbestrol and 0 per cent of 20 male controls. Abnormal findings on physical examination combined with severe sperm abnormalities (Eliasson score more than 10) were found in 23 per cent of the male patients exposed to diethylstilbestrol versus none of the male controls. Cytologic examinations revealed no malignant cells from urine samples before and after prostatic massage or ejaculation, prostatic fluids and aspirates from epididymal cysts.