The human p53 gene, a putative tumor suppressor gene, has a polymorphism in amino acid residue 72. We recently developed a method of detecting codon 72 polymorphism in this gene by digestion of polymerase chain reaction-amplified DNA using an allele-specific restriction endonuclease. This polymorphism allows the identification of loss of heterozygosity for the coding region of the p53 gene in limited tissue samples in a short time without using radioactive materials. We examined 33 patients with renal cell carcinoma and 29 with bladder cancer; heterozygosity in the p53 gene was lost in 60% (6 of 10 cases) and 73% (8 of 11 cases) of the renal and bladder tumors, respectively. Additionally, the assay's sensitivity could be improved by using DNA extracted from frozen sections of the tumors. Because the proportions of tumor cells and nontumor cells could be assessed by microscopic evaluation of the frozen sections, we were able to minimize contamination from nontumor cells, which occasionally causes false readings of retained heterozygosity. This simple and sensitive method for detecting loss of heterozygosity in the p53 gene makes it possible to rapidly screen a large number of tissue samples and has the potential to be a useful diagnostic tool for a wide variety of human neoplasms.