We reviewed 519 cases of clear-cell adenocarcinoma of the vagina and cervix identified by the Registry for Research on Hormonal Transplacental Carcinogenesis of the University of Chicago through June 30, 1985. In 60 percent of all cases the patient's mother had received diethylstilbestrol during pregnancy. An additional 12 percent of all mothers had been treated with another hormone or with an unidentified medication. Ninety-one percent of the cases in diethylstilbestrol-exposed women were diagnosed when the patient was between the ages of 15 and 27. The median age at diagnosis was 19.0 years. The risk that clear-cell adenocarcinoma will develop in an exposed female from birth through age 34 is 1 case per 1000 women. The temporal pattern of occurrence of clear-cell adenocarcinoma corresponds closely with that of the use of diethylstilbestrol for pregnancy support in the United States. The rarity of this tumor among exposed women suggests that diethylstilbestrol is not a complete carcinogen and that some other factor is also involved in the pathogenesis of clear-cell adenocarcinoma of the vagina and cervix.