AbstractLeiomyomas, adenomyosis, and endometriosis are reported to be risk factors for endometrial carcinoma (EC), and adenomyosis and endometriosis also for ovarian carcinoma (OC). We aimed to describe the prevalence of these conditions in EC patients with or without an OC diagnosis, and to investigate their relationship with EC risk and prognostic factors in these patients. We evaluated the co-existence of these three conditions in 1399 EC patients, and compared the prevalence of epidemiological risk factors and tumor prognostic features in patients with each condition versus not. Prevalence of conditions was also assessed in the subset of patients with prior/concurrent OC. The observed coexistence of leiomyomas, adenomyosis and endometriosis significantly deviated from that expected (P = 1.2 × 10⁻⁸). Patients were more likely to: report a younger age at menarche (P_Trend = 0.004) if they had leiomyomas; have used oral contraceptives (P = 6.6 × 10⁻⁵) or had ≥2 full-term pregnancies (P_Trend = 2.0 × 10⁻⁹) if they had adenomyosis; be diagnosed with EC at younger age (P = 5.0 × 10⁻¹¹) if they had endometriosis. Patients with prior/concurrent OC were more likely to be diagnosed at younger age (P = 5.0 × 10⁻⁵), have endometriosis (P = 9.9 × 10⁻⁷), and present with higher stage EC (P_Trend = 6.6 × 10⁻⁵). These findings justify further consideration of these gynecologic conditions as independent risk and prognostic factors for EC.