The occurrence of vaginal clear cell adenocarcinoma in young women following exposure in utero to diethylstilbestrol (DES) is now well documented. In addition to this carcinogenic potential. DES has been shown to be teratogenic. In females, the DES-related malformations include vaginal adenosis, transverse ridges of the vagina or cervix and uterine abnormalities. Although no neoplasms have been observed in DES-exposed males, malformations of the epididymis, testes and phallus are relatively common and may result in infertility. The carcinogenic mechanism of DES may be either a direct induction of malignant potential in vaginal cells or a teratogenic effect causing ectopic Müllerian epithelium which could be exposed later to mutagenic agents in the vagina. The absence of malignancy in DES-exposed males may favor the latter hypothesis since male Müllerian remnants are internal structures and thus would not be exposed to surface carcinogens.