The occurrence of columnar epithelium in the vagina (vaginal adenosis) in young women with intrauterine exposure to diethylstilbestrol (DES) during the first trimester of pregnancy was observed in 231 patients (82 per cent of 280 cases who underwent colposcopic study). Extension of columnar epithelium onto the portio of the cervix was present in the remaining 18 per cent of the cases. Abnormal colposcopic findings were present in the transformation zone in 96 per cent of the patients with vaginal adenosis. Directed biopsy revealed four cases of vaginal and/or cervical squamous carcinoma in situ (CIS), two cases of severe dysplasia, five cases of moderate, and 29 cases of mild dysplasia. The prevalence of CIS in DES-exposed girls (1.4 per cent) was nearly five times the prevalence rate of CIS in a control group of 5,808 DES-unexposed women (0.44 per cent). This finding correlates well with the hypothesis that the genesis of squamous intraepithelial neoplasia is specifically related to the extent and surface area of the vaginal transformation zone. An unusual case of invasive squamous carcinoma in a DES-exposed young girl is presented, which represents the initial observation of this association to date.