Between 1975 and 1990, 79 patients with Stage I epithelial ovarian cancer were treated at Massachusetts General Hospital. Patients were identified from the tumor registry and medical records were retrospectively reviewed. Pathological slides were evaluated for the presence of endometriosis, specifically looking for malignancy arising in endometriosis. Evidence of endometriosis was found in 22 of the 79 cases (28%). In the 23 cases of endometrioid histology, 9 cases (39%) were associated with endometriosis and, in the 17 cases of clear cell tumors, 7 (41%) were associated with endometriosis. All 8 cases of mixed histology had clear cell and/or endometrioid components and 4 cases (50%) were associated with endometriosis. Endometrioid adenocarcinoma accounted for 41% of the tumors associated with endometriosis, clear cell carcinoma 31%, mixed (endometrioid and/or clear cell types) 18%, and other types 9%. Among the 22 patients with associated endometriosis, we found 7 carcinomas (32%) arising in endometriosis. In these 7 cases a spectrum of benign and atypical endometriosis with a transition to clear cell or endometrioid adenocarcinoma were identified. These premalignant changes were characterized by cytologic atypia and architectural proliferation. Endometriosis was frequently encountered among patients with Stage I epithelial ovarian cancer of endometrioid and clear cell histologies. Endometriosis may play a role in the pathogenesis of some early stage malignant ovarian epithelial neoplasms.