Sixty-eight cases of sacrococcygeal teratoma were reviewed and graded according to the quantity of immature tissue present. Seventy-five percent were benign (grade 0), 11.8% immature (Grades 1-3), and 13.2% malignant. Although the immature component in most tumors was neuroepithelial, in two cases it was exclusively renal. There were no instances of local recurrence or metastasis of immature elements. The malignant lesions were all endodermal sinus tumor, stained positively for alpha-fetoprotein and were uniformly fatal, with an average duration of survival of 8.9 months. Early adequate treatment of sacrococcygeal teratomas results in a favorable prognosis. This experience is different from that reported for immature ovarian teratomas. Immature metanephric tissue in sacrococcygeal teratomas should be interpreted the same way as immature neuroepithelial elements. Immature sacrococcygeal teratomas should be separated from tumors containing malignant elements because of their vastly different prognoses.