Two hundred eight patients with confirmed or suspected diethylstilbestrol exposure were examined colposcopically at two separate medical centers. Cervicovaginal ridges were present in 90 (43.3%) and ectopy was found in 188 (90.4%) of the patients. Evidence of vaginal adenosis was present in 122 (58.6%) of the cases. Seventy-four percent of the patients had abnormal colposcopic findings: 15.4% having columnar epithelium on the surface of the vagina and 66% having abnormal transformation zones (white epithelium, punctation, mosaic). Twenty patients (9.6%) were initially identified histologically as having squamous dysplasia. Subsequent review of the histologic material in these cases could document only two cases of significant squamous dysplasia, both severe, the remainder having immature, atypical metaplasia or possibly very mild dysplasia. This finding emphasizes the problems encountered in histologically differentiating squamous neoplasia from the peculiar metaplasia found in these patients, thereby making it difficult to establish whether these patients are at increased risk for the development of squamous neoplasia.