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Summary: Background: Ovarian cancer is commonly fatal and incidence has persistently risen in Taiwan over the past 20 years. Prevention strategies, however, are limited. Pelvic inflammatory disease (PID) has been suggested to increase the risk of developing ovarian cancer, but the results of studies have been inconsistent. Therefore, we investigated whether PID increases the risk of developing ovarian cancer in a large, nationwide cohort. Methods: From the Longitudinal Health Insurance Database 2005 (LHID2005) in Taiwan, we obtained data for women aged 13–65 years for whom a diagnosis of PID, confirmed by multiple episodes, had been recorded between Jan 1, 2004, and Dec 31, 2005. We also obtained data for two controls per patient, matched for age and the year of first entry into the LHID2005. All patients were followed up from the date of entry in the LHID2005 until they developed ovarian cancer or to the end of 2006, whichever was earlier. We used Cox''s regression models to assess the risk of developing ovarian cancer, with adjustment for age, comorbid disorders, and socioeconomic characteristics. Findings: We identified 67 936 women with PID and 135 872 controls. Among these 90 had developed ovarian cancer during the 3-year follow-up period (42 patients with PID and 48 controls, incidence 2·78 and 1·44 per 10 000 person-years, respectively). The adjusted hazard ratio for ovarian cancer in patients with PID was 1·92 (95% CI 1·27–2·92) compared with controls, which rose to 2·46 (1·48–4·09) in women who had had at least five episodes of PID. The adjusted hazard ratio was slightly higher for women aged 35 years or younger with PID than in older women with PID (2·23, 1·02–4·79 vs 1·82, 1·10–3·04). Interpretation: We found an association between PID and ovarian cancer. PID might, therefore, be a useful marker for ovarian cancer, and early treatment could help to improve prognosis. Whether pelvic inflammation itself accelerates the growth of ovarian cancers or affects cancer-cell differentiation in ways that adversely alter prognosis needs to be investigated. Funding: None. [Copyright &y& Elsevier]