An increased frequency of various genitourinary anomalies, infertility, and testicular cancer among males has been reported to follow intrauterine exposure to diethylstilbestrol, but not all studies have confirmed an association. This study was designed to determine whether a cohort of males exposed in utero to diethylstilbestrol had a higher frequency of urogenital abnormalities than an unexposed cohort. Biases in selection of exposed and control participants were minimized. Of 828 exposed and 676 control men studied by medical-record review, 265 exposed men and 274 controls also underwent a special clinical examination. Overall, the data suggest that diethylstilbestrol exposure of males in utero did not increase their risk of genitourinary abnormalities, infertility, or testicular cancer. Previously reported increased frequencies of these abnormalities in diethylstilbestrol-exposed men may have resulted from selection biases or differences in diethylstilbestrol use, or both.