Twenty-nine pediatric immature teratomas were reviewed to determine the frequency and clinical significance of p53 expression. Tumors were stained for p53 expression by immunohistochemistry and results were correlated with the presence of other germ cell tumor elements and with outcome. Sequencing of p53 for mutations was performed on positive cases. Eighteen cases showed widespread positive p53 staining of the immature teratoma elements, 9 showed staining only in very occasional cells, and 2 cases showed no staining. Of the 18 positive cases, 5 recurred. All five were pure immature teratomas at diagnosis. Four recurred as immature or mature teratoma and one as a sarcoma; all except one showed frequent cells positive for p53 in the recurrent tumor. Another 5 of the 18 diffusely positive cases contained immature teratoma as well as other malignant germ cell elements at diagnosis; none of these recurred. None of the remaining eight cases with frequent positive cells, the nine cases with occasionally positive p53 staining, or the two cases with no staining recurred or demonstrated other germ cell tumor elements. We conclude that p53 expression is not unusual in immature teratoma and diffuse p53 immunopositivity is associated with recurrence or the presence of malignant elements in approximately 50% of cases. In only 1 of 29 cases tested was p53 immunopositivity associated with mutations in the p53 gene; hence, overexpression in the majority of cases is presumed to reflect increased half-life of the protein from undetermined stabilizing factors. Expression of p21, a p53 target gene, was only focal, suggesting impaired transcriptional activation by p53. The finding of frequent p53-positive cells in immature teratoma should prompt a search for malignant elements within the tumor and affected patients should be followed closely for evidence of recurrence.