A false aneurysm of the superior gluteal artery with compression of the sciatic nerve is reported in an 18-year-old man who was recovering from multiple injuries sustained in a bicycle accident. Rapidly increasing pain and swelling in the leg, with a rapid fall in the hemoglobin level and weakness of dorsiflexion and inversion of the foot were not compatible with the initial diagnosis of abscess. A selective iliac arteriogram revealed the true diagnosis. The gluteal artery was ligated and the false aneurysm oversewn through a posterior, extrapelvic approach. An abdominal, retroperitoneal approach would have been better but was precluded by injuries to the intrapelvic viscera and left-sided colostomy. False aneurysm of the gluteal artery is a life-threatening condition and must be diagnosed early and treated promptly to ensure a successful outcome.