Endometriosis of the abdominal wall typically occurs as a painful mass in a lower abdominal incision from previous cesarean section or hysterectomy. Most patients are young and in their active reproductive years. The histologic diagnosis requires a combination of either endometrial-like glands, endometrial stroma, or hemosiderin pigment. The diagnosis must be considered in any woman with an abdominal wall mass and a history of transabdominal gynecologic surgery. Wide excision offers the best chance to prevent recurrence.