During the last 6 years, there has been a marked increase in the incidence of vaginal and cervical clear cell adenocarcinomas occurring in young females. A relationship between maternal ingestion of synthetic estrogens occurs in about 75% of these cases. DES was used in a variety of pregnancy complications between 1940 and 1970 with the peak years being 1945 through 1960. There is no explanation for the increased number of cases occurring in patients whose mothers did not ingest synthetic estrogens. The number of women who took these drugs during pregnancy and their at-risk daughters is unknown. Vaginal adenosis is an associated finding in about 90% of the vaginal and 30% of the cervical adenocarcinomas. The reported incidence of adenosis in DES-exposed offspring varies from 30 to 90%. Gross cervicovaginal abnormalities occur in about 20% of the exposed patients. The relationship between adenosis and clear cell adenocarcinoma is not clear; however, many investigators feel adenosis is a precursor. Both adenosis and the clear cell tumors are of Müllerian origin, and it appears that any dose of synthetic estrogens ingested prior to the 18th week of pregnancy alters the normal embryogenesis of the vagina.