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STUDY QUESTION Is ovarian or extra-ovarian endometriosis associated with an increased risk of ovarian cancer and borderline ovarian tumours (BOT)? SUMMARY ANSWER We found a 3- to 8-fold increased risk of ovarian tumours associated with endometriosis: the magnitude of the risk increase depended on the definition of endometriosis. WHAT IS KNOWN ALREADY There is increasing evidence of an association between endometriosis and increased risk of ovarian cancer. However, most reports were based on self-reported diagnosis of endometriosis. STUDY DESIGN, SIZE, DURATION We conducted a nationwide historic cohort study among women with subfertility problems between 1980 and 1995. For this analysis we selected all cohort members with endometriosis, and a comparison group of subfertile women (male factor or idiopathic) without endometriosis (total cohort of 8904 women). Median follow-up time was 15.2 for the entire study population. PARTICIPANTS/MATERIALS, SETTING, METHODS For this analysis we selected all cohort members with (n = 3657) and without (n = 5247) evidence of endometriosis. Seventy-eight per cent of diagnoses of endometriosis were confirmed by pathology report, and 22% was self-reported endometriosis (positive predictive value of 73%). We linked the cohort with the Dutch Pathology Database and the Netherlands Cancer Registry to assess the occurrence of ovarian cancer and BOT between January 1989 and June 2007. MAIN RESULTS AND THE ROLE OF CHANCE We observed a substantially increased risk of all ovarian malignancies combined in women with endometriosis when we based the definition of endometriosis on self-report, medical records information at subfertility treatment and/or the nationwide pathology database (hazard ratio (HR) 8.2; 95% confidence interval (CI) 3.1–21.6). The HR associated with endometriosis was 12.4 (95% CI 2.8–54.2) for ovarian cancer and 5.5 (95% CI 1.5–20.2) for BOT. When we excluded information from the pathology database, HRs were 3.0 (95% CI 1.5–6.1) for all ovarian tumours, 4.3 (95% CI 1.6–11.2) for ovarian cancer and 1.9 (95% CI 0.6–5.8) for BOT. Both ovarian and extra-ovarian endometriosis carried a significantly increased risk for ovarian cancer and BOT. LIMITATIONS, REASONS FOR CAUTION We did not have information on oral contraceptive use and parity for 23.4 and 3.4%, of women in the analytic cohort, respectively. Furthermore, a limitation of our study, and also of other studies, is that the date of diagnosis of endometriosis is usually made long after the onset of the disease. Also, the number of cases in the cohort is small (n = 34), resulting in wide CIs. WIDER IMPLICATIONS OF THE FINDINGS The fact that endometriosis was assessed before diagnosis of ovarian malignancy and the high degree of medical confirmation in our study likely contribute to the validity of our estimate of a 3- to 8-fold increased risk of ovarian tumours associated with endometriosis. The risk of ovarian malignancies associated with endometriosis was much higher in analyses including information on endometriosis from the nationwide pathology database, implying that risk estimates from studies using self-reported information on endometriosis may be too low due to non-differential misclassification bias. STUDY FUNDING/COMPETING INTEREST(S) None. TRIAL REGISTRATION NUMBER None. [ABSTRACT FROM PUBLISHER]