One hundred ninety-four cases of ovarian endometriosis, encountered during a 3-year period, were studied. In addition to endometrial-type epithelium and stroma that were present in each of these cases, oviduct-type epithelium was demonstrated in 26, papillary epithelial-lined projections in 7, and hobnail cells in 36 patients. Forty-three cases showed reactive epithelial changes, probably due to inflammation and regeneration. There were 4 cases of ade nomalous hyperplasia. Severe epithelial atypism was seen in 7 patients (3.6%), I in conjunction with adenomatous hyperplasia. Although the areas of severe epithelial atypism in ovarian endometriosis may be of reactive origin, one must also consider the possibility that in some instances these atypical changes may constitute a neoplastic potential.