Squamous cell abnormalities of the vagina and cervix were evaluated in 1424 women exposed to diethylstilbestrol (DES) in utero. The prevalence of dysplasia was 2.1% and the incidence 0.85/100 person-years of followup. The dysplastic epithelial changes were almost always mild in women with no prior history of dysplasia and was slightly more frequent in the cervix than the vagina. Severe dysplasia and carcinoma in situ (CIS) were encountered only in those subjects specifically referred because of those abnormalities. The most common problem in the diagnosis of these squamous cell changes was the misinterpretation of mature and immature metaplastic cells for dysplastic squamous cells. Discordance between biopsy and cytology was common-place in the detection and followup of dysplasia, especially when it was mild. There were no instances in the study where cytology and biopsy samples from the vagina were both abnormal concurrently. Colposcopically directed biopsies did not increase the frequency of confirmation of cytologic findings. These data suggest that both cytology and biopsy of abnormal segments of the vagina and cervix remain an integral part of the examination of the DES-exposed female during long-term follow-up studies.