Endodermal sinus tumors are an uncommon malignancy in children and occur most often in the gonads and sacrococcygeal region. In these locations, thy are found as primary tumors ; in teratoma they form part of the malignant component. We present CT and MR imaging findings in two cases of endodermal sinus tumor occurring after the resection of a mature benign cystic sacrococcygeal teratoma during the neonatal period. Awareness of the transformation of a benign cystic teratoma to a different type of malignant germ cell tumor may be helpful in the evaluation of sacrococcygeal masses.