Sacrococcygeal teratomas can be diagnosed prenatally with a detailed ultrasound examination of the fetal anatomy. The majority of these teratomas are solid or mixed cystic and solid external caudal masses. A conclusion drawn from the literature is that only a small percentage are entirely cystic and few are completely internal or presacral without external component (Type IV). This report describes an unusual case of a large completely internal cystic sacrococcygeal teratoma identified prenatally by ultrasonography at 32 weeks' gestation. The differential diagnosis for this unique presentation of sacrococcygeal teratoma is different from that considered for predominantly external complex tumors.