We report a case of endometrioma in the pelvic cavity. A 41-year-old woman presented with a peculiar periodic sciatica associated with her menses. Computed tomography showed an oval-shaped mass in the pelvic cavity compressing the right sciatic nerve. To confirm the diagnosis, hormonal therapy through nose dropping of buserelin acetate was given to stop menstruation. After treatment, her symptoms disappeared completely. When a sciatica is closely related to menses, orthopedic surgeons should consider cyclic sciatica resulting from endometrioma as a differential diagnosis.