STUDY OBJECTIVE: To report on the presence and rate of endometriosis in hydatid cysts of Morgagni found at the time of excision surgery for endometriosis and to describe any association of endometriosis in hydatid cysts of Morgagni with preoperative or operative factors. DESIGN: A retrospective cohort study (Canadian Task Force Classification II-2). SETTING: The Center for Endometriosis at Saint Louis University, a tertiary referral center for endometriosis. PATIENTS: Women who underwent optimal excision surgery for suspected endometriosis because of chronic pelvic pain and/or infertility and who also had hydatid cysts of Morgagni removed at the time of surgery when found. INTERVENTIONS: Preoperative and operative data were collected prospectively. MAIN OUTCOME MEASURES: The rate of endometriosis in hydatid cysts of Morgagni. Secondary measures included are the rate of hydatid cysts of Morgagni in patients with pelvic pain or infertility with and without endometriosis in the cysts. RESULTS: The overall prevalence of endometriosis in hydatid cysts of Morgagni was 11.3%. Patients with pelvic pain had a higher rate (although not statistically significant) of hydatid cysts of Morgagni compared with those without pain (21.1% vs 12.5 %, p = .54). Patients with infertility had a higher rate of hydatid cysts of Morgagni compared with those without infertility (38.1% vs 16.7%, p < .001), and there was a higher rate of endometriosis in the hydatid cysts of Morgagni in patients with infertility compared with those without (11.1% vs 0.0%, p