The occurrence of ovarian endometriosis has been studied in 990 cases of primary epithelial carcinoma of the ovary. In 357 serous and 155 undifferentiated lesions no example of endometriosis was found and only one example in 203 mucinous tumours. In the 205 cases of endometrioid carcinoma the incidence of endometriosis was 9.3%. In the 36 cases of clear cell carcinoma and the 23 cases of mesonephroid carcinoma with tubular pattern the incidence was significantly higher, 19.4% and 25.9% respectively. The incidence in endometrioid tumours was significantly higher in those cases in clinical stage I b as compared with other stages. Possible histogenetic relationships between endometriosis and ovarian carcinoma are discussed. Follow-up study of these patients clearly showed that it is of the utmost importance to attempt complete removal of all tumour tissue at the primary operation. Preoperative irradiation using super-voltage technique is advocated.