Objective: To describe a rare extrauterine mass of an unknown origin that was composed of smooth muscle with a central cavity lined by endometrium, which resembled a uterus. Design: Case report. Setting: Academic teaching hospital. Patient(s): A 17-year-old girl with severe dysmenorrhea was found to have a uterus-like mass composed of smooth muscle and lined by endometrium in the left broad ligament. Intervention(s): Abdominal surgery. Result(s): The mass was successfully removed surgically, and her dysmenorrhea resolved. Histopathological examination revealed that the uterus-like mass was composed of smooth muscle with a cavity lined by endometrial glands and stroma. Conclusion(s): The histogenesis of a uterus-like mass is theorized to be a Müllerian system anomaly, metaplastic change, and/or heterotopia. We believe metaplasia is the most likely cause in this case. Alhough rare, it should be considered in the differential diagnosis of dysmenorrhea of uncertain etiology. Crown Copyright © 2010.