Twins were diagnosed by ultrasound in a 29-year-old woman. The cesarean delivery revealed at first a mature girl with a large benign sacrococcygeal teratoma and then a healthy boy. X-ray and CT examinations of the teratoma showed pelvic and lower limb bones. A mature teratoma-tridermoma (weight 475 g) containing the underdeveloped lower half of a human body was confirmed during surgery and verified morphologically. The fraternal twins, i.e., the girl operated upon and her brother, have been followed for 5 years and are without any complaints. They have normal neurological function.