The current evidence associating maternal ingestion of diethylstilbestrol (DES) with the subsequent appearance of clear-cell adenocarcinoma of the genital tract has been presented. Thus far, these cancers are rare among exposed females. DES, dienestrol, and hexestrol, all nonsteroidal synthetic estrogens, have been implicated. The tumors have appeared primarily in exposed subjects over the age of 14 yr with the median ranging from 18 to 19 yr. Nonneoplastic abnormalities have been observed commonly in the exposed, including vaginal adenosis, cervical erosion (ectropion), and transverse vaginal and cervical ridges. The highest incidence of vaginal adenosis was noted among those whose mothers began DES in early pregnancy, and no cases were observed among those whose mothers began treatment after the 18th wk. Current evidence suggests that these alterations as well as the carcinomas are of mullerian origin.