One thousand consecutive cases of surgically proven endometriosis were reviewed to evaluate the frequency and types of pelvic cancers that were associated with ovarian and extraovarian endometriosis. The frequency and types of histologic abnormalities present in the eutopic endometrium when cancers were noted in endometriosis were also evaluated. In the large subset of cases for which the authors were the primary pathologists and all foci of endometriosis were recorded, the frequency of malignancy was 10.8%. In contrast, the frequency was only 3.2% in cases diagnosed by others previously in our institution. Cancers were more commonly found in ovaries when endometriosis was present in that ovary (5%) compared to when endometriosis was present at other sites (1%). Clear cell and endometrioid carcinomas were the malignancies most commonly seen in ovaries containing endometriosis, while clear cell adenocarcinoma and adenosarcoma were most commonly seen in conjunction with extraovarian endometriosis. The association of endometriosis with endometrioid and clear cell carcinoma was much stronger than that of serous and mucinous tumors (p < .01). Concurrent endometrial pathology was commonly seen in cases of malignant transformation of endometriosis (32% of cases).