In the late 1960's when a series of adolescent girls with adenocarcinoma of the vagina presented themselves to our hospital, we did not immediately suspect the cause. Previous experience with radical hysterectomy and with vaginal reconstruction had prepared us technically to treat them successfully. Then a clue to etiology from one mother's observation regarding DES as a pregnancy supportive medication was quickly and conclusively converted into fact by an investigation with case controls. Rapid expansion of our knowledge came about through information accumulated in a Registry, and untoward effects other than cancer soon came to notice as young asymptomatic women presented themselves for examination because of known fetal exposure. The dominant event is clearly recognizable as teratogenic, resulting in anomalous development of the vagina and cervix. Although the appearance of clear cell adenocarcinoma in a small fraction of DES‐daughters is a consequence, the role of DES in its carcinogenesis is still unproved.