Three cases of clear cell carcinoma of the cervix and vagina in progeny of women administered diethylstilbestrol (DES) were studied ultrastructurally. The patients were young and had a positive history of maternal ingestion of DES during pregnancy. Similar ultrastructural features were observed in all cases. The neoplastic cells were arranged in solid nests or glands. Ultrastructural findings in these cases were similar to those of previously reported clear cell carcinomas of the ovary and endometrium, which seems to indicate a common Müllerian origin11, 16 as opposed to a mesonephric origin. 10 A transition between vaginal adenosis and the neoplasm was not observed.