In order to better understand the clinical presentation and biologic behavior of ovarian carcinomas arising in endometriosis, we performed a historical cohort study of all women with endometrioid adenocarcinoma of the ovary (ECO) diagnosed between 1979 and 1991 at our institutions. A review of pathology reports determined the presence or absence of coexisting endometriosis. Cancers adjacent to endometriosis on the same ovary or arising within endometriosis were labeled endometriosis-associated endometrioid adenocarcinoma (EAEA), while all others were considered typical endometrioid adenocarcinoma (TEA). Associations between tumor type and clinicopathologic variables were analyzed by chi 2 and Fisher's exact tests as indicated. Disease-free interval (DFI) and overall survival were estimated using the Kaplan-Meier method. The independent prognostic significance of clinicopathologic variables was determined by multivariate analysis using the Cox proportional hazards regression model. Of 91 ECO patients identified, 63 (69%) had TEA and 28 (31%) had EAEA. Significant differences between TEA and EAEA existed for age at diagnosis (greater than 55 years; 56 vs 32%, P = 0.039), nulliparity (19 vs 46%, P = 0.007), stage (I and II combined; 37 vs 70%, P = 0.004), and disease status at completion of primary surgery (complete tumor resection; 47 vs 70%, P = 0.04). Synchronous atypical endometrial hyperplasia or uterine carcinoma was found in 7/63 (11%) TEA versus 7/28 (25%) EAEA cases (P = 0.054). Estimated 5-year DFI by life table analysis was significantly longer in the EAEA than in the TEA cohorts (57 vs 25%, P = 0.02); however, the 5-year survival difference was not significant (59 vs 45%, P = 0.18). Multivariate analysis identified only stage as an independent prognostic factor in predicting both DFI and survival. In conclusion, women with EAEA are significantly younger, present with earlier stage disease, and have a longer disease-free survival than those with TEA. These factors may reflect a more favorable biologic behavior of ECO when arising in association with endometriosis.