One hundred forty-seven cases of ovarian endometriosis, encountered from 1976 to 1999 at Tsukuba University Hospital, were studied to clarify the incidence of malignant transformation. There were 18 cases (12.2%) of atypical endometriosis, among which we found a case (5.6%) of ovarian cancer arising from endometriosis not diagnosed before surgery. This is accounted for 0.7% of all ovarian endometriosis cases. Because the incidence was equal to that of the previous reports, it is most likely that the malignant change in ovarian endometriosis occurred in 0.7% of this disease.