Sixteen cases of clear cell adenocarcinoma of the cervix and vagina in young women were studied by electron microscopy and the observations were compared with the light microscopic appearance. Ultrastructurally, the neoplastic cells were found to be of the same basic type, regardless of whether light microscopy disclosed a solid or tubulocystic tumor pattern, and whether the cells were clear, hobnail, flat or of a nonspecific müllerian appearance. Cytoplasmic glycogen and short, blunt, surface villi were the outstanding features. Less consistent findings included prominent Golgi apparatuses and numerous mitochondria. The fine structural characteristics were similar to those reported for clear cell adenocarcinoma of the endometrium and ovary in older women. This study disclosed no difference between 14 tumors that arose in patients with confirmed histories of prenatal exposure to diethylstilbestrol (DES) or dienestrol, and reported cases of clear cell carcinoma in which there were no histories of such exposure. No specific ultrastructural features could be identified as prognostic indices for the neoplasms.