Ten cases of malignant tumors arising in foci of gonadal and extragonadal endometriosis are reported and added to 195 previously reported cases from the English literature. The ovary was the primary site in 165 (78.7%) of the cases, whereas extragonadal sites represented 44 (21.3%). Endometrioid adenocarcinomas accounted for 69% of the lesions, clear-cell carcinomas 13.5%, sarcomas 11.6%, and rare cell types 6%. Extragonadal lesions were mostly endometrioid tumors (66%) and sarcomas (25%). Tumors arising in endometriosis were predominantly low grade and confined to the site of origin. Radiation therapy was often able to control completely tumors limited to the pelvis, but was not beneficial in metastatic disease. Only one patient had a response to chemotherapy. Fourteen patients received postoperative progestin therapy, with a 77% 5-year survival. Follow-up has been reported in 86 patients. The tumor was either confined to the ovary (57), confined to the extragonadal site of origin (11), or spread throughout the peritoneal cavity (18). With each of these situations, the 5-year survival was 65, 100, and 10%, respectively. Fourteen patients had malignant transformation in endometriosis associated with presumed estrogenic stimulation; most lesions (69%) were well differentiated and the 5-year survival was 82%. After surgical resection, we recommend that progestin therapy be included in the treatment of cancer arising in endometriosis. The actual frequency of malignancy arising in endometriosis may be higher than reported.