This prospective study of 271 females believed to have had prenatal exposure to stilbestrol documents the cytologic findings in separate vaginal and cervical preparations. In 233 patients, direct circumferential scrapings of the upper vaginal wall were taken after potentially contaminating mucous secretions had been removed. These specimens were considered evaluable for the cytologic detection of vaginal adenosis. Cellular evidence of vaginal adenosis was observed in 105 (45%) of these patients as determined by the identification of columnar cells and/or metaplastic squamous cells in the vaginal samples. Columnar cells were observed in 87 (37%) of the 233 patients. Metaplastic squamous cells accompanied the columnar cells in 54 cases. In an additional 18 patients (8%) metaplastic squamous cells were the only cytologic evidence of adenosis. Similar cellular changes were observed in 88 per cent of the vaginal samples from 25 patients with biopsy-proven vaginal adenosis. Malignant cells were not found in any of the cytologic or histologic specimens. Dysplastic squamous cells, however, were present in cytosmears from four patients. Biopsies from two of these patients confirmed the diagnosis of squamous dysplasia. Dysplasia also was identified by biopsy in four others. We believe cytology is a valuable aid in the evaluation of patients with a history of intrauterine exposure to stilbestrol. Vaginal adenosis as well as neoplastic and dysplastic lesions can be reflected in properly obtained cellular specimens.