A 33-year-old woman with a 2-year history of swelling and pain in her buttock and left thigh fluctuating with her menstrual cycle who was becoming progressively disabled was referred to the department of orthopedics. Magnetic resonance imaging (MRI) detected a left buttock lesion of 3 x 2 cm that was initially diagnosed as muscular-fiber laceration with associated hematoma. The worsening of her symptomatology required an ultrasound-guided biopsy of the lesion that revealed endometriosis. Laparoscopy showed the pelvis to be free of gross disease. Hormonal suppression by means of gonadotropin-releasing hormone analog therapy proved adequate in temporarily alleviating symptoms. A year later the patient underwent surgical excision of the buttock lesion, which was effective in alleviating her symptoms for a short period of 10 months. A 1-year follow-up MRI revealed several small endometriotic foci, located among piriformis and obturator internus muscle fibers, which were considered not suitable for surgical removal. The patient is currently on a drug regime for pain management. However, she has experienced permanent muscular damage on her left buttock including significant omolateral gluteus strength reduction, functional impairment (inability to rotate laterally or bend her left leg), and the assumption of an antalgic gait while walking. Because of impairment in her deambulation capability, total physical invalidity was agreed for her by the National Health Care Services.