A study was undertaken to evaluate the histologic alterations associated with two or the most abnormal colposcopic findings, mosaicism and punctation, that are commonly found in the vaginas and cervices of young women who have been exposed prenatally to diethylstilbestrol (DES). Four-fifths of 215 biopsy specimens from 171 exposed subjects with mosaicism or punctation disclosed metaplastic squamous epithelium, the presence of which is thought to reflect the repair of vaginal adenosis and cervical ectropion. Hyperkeratosis was observed in one-fifth of the specimens and was more frequent in lesions colposcopically graded I than II. Mild dysplasia was encountered in only one specimen. The findings indicate that the presence of squamous cell dysplasia cannot be predicted by the finding of Grade I or II mosaicism or punctation in the DES-exposed female despite the fact that these abnormal colposcopic patterns are associated with dysplasia in about 10% of cases when encountered in the cervix of the unexposed woman.