In six women with a preoperative diagnosis of incarcerated hernia, surgical exploration of the groin revealed inguinal endometriosis and no hernia. The affected structure was always the extraperitoneal portion of the round ligament. Six of the seven lesions were on the right. Intraperitoneal endometriosis was demonstrated in all patients. Catamenial pain was the pathognomonic symptom in the differential diagnosis of the inguinal mass. Gynecologists and surgeons should bear in mind the possibility that endometriosis may be the cause of symptoms of a suspected incarcerated inguinal hernia. Should this disease be detected at inguinal exploration, a laparoscopy is indicated during the same operation.