In a colposcopic evaluation of DES-related genital abnormalities, biopsy-proven adenosis was detected in the vaginal walls or hood in 84.5% of 220 women. Patient selection (DES history only vs prior examination) did not significantly influence the detection rate for adenosis. However, the detection rate of adenosis in the vaginal walls showed highly significant correlations with the presence of cervical ectopy and/or hood and with the colposcopic appearance of the area biopsied. Using colposcopy, the columnar pattern had a diagnostic accuracy of 94.4%; mosaic pattern, 85.5%; and white appearance, 68.0%. Differences in the histologic features of mucosal surface glandular tissue, squamous metaplasia, and nonglycogenated squamous mucosa were related to specific colposcopic patterns and helped to account for the variation in the accuracy of the three appearances in detecting adenosis. Biopsy specimens of the hood were not necessary to achieve a high yield of adenosis although we obtained evidence that the hood contains microscopic features typical of vaginal adenosis. Epithelial atypicality in the vagina (moderate squamous cell dysplasia) was demonstrated in only 1 patient.