The anaplastic variant of Wilms' tumor is regarded as the result of tumor progression of the more common classic Wilms' tumor. Anaplasia is rare and occurs in only 4.5% of tumors. Three anaplastic Wilms' tumors in our collection were examined in comparison with 10 classic Wilms' tumors for p53 expression by immunohistochemical techniques and Northern blot analysis, and their p53 gene structure was determined by single-stranded conformation polymorphism and sequence analysis. All classic tumors contained a wild-type p53 gene and expressed marginal levels of protein as expected for normal p53. In contrast, three out of three anaplastic tumors demonstrated evidence of p53 alterations consistent with a role of p53 in tumor progression. One of the anaplastic mutants (W4) did not express protein or p53 mRNA. Its apparently normal immunophenotype would have disguised the mutated nature of p53, which was detected only by mRNA and sequence analysis. The second anaplastic mutant (W16) contained normal levels of p53 mRNA, but overexpressed the protein in a fashion typical of mutated p53. The same immunophenotype was displayed by fixed primary tissue of the third anaplastic tumor (W17), but p53 mutation could not be confirmed for lack of frozen primary material. The present study emphasizes the importance of p53 function in the anaplastic progression of Wilms' tumor and the risk of error in assessing normal p53 function using a single methodology.