The p53 tumor suppressor gene is often mutated in various human cancers and a common polymorphism is known at codon 72 of exon 4, with two alleles encoding either arginine (CGC) or proline (CCC). Association of this polymorphism with any human cancer susceptibility has yet to be clarified. We have conducted a case-control study in Japan on the distribution of the three genotypes with 191 lung cancer patients, 152 control patients with non-cancerous pulmonary diseases and 115 colorectal cancer patients. The genotypes were examined by PCR using DNA samples from peripheral blood lymphocytes. Frequency distributions of the three genotypes were quite comparable with each other among groups, with allelic frequencies of approximately 60% for arginine and 40% for proline. The genotypic frequencies in lung cancer patients, however, were largely different between smokers and non-smokers (chi 2 = 13.5, df = 2, P < 0.001). Compared with the control and colorectal cancer patients a significant difference in genotypic frequency was observed only in non-smoker lung cancers (chi 2 = 10.9, df = 2, P < 0.01), with an excess of Arg/Arg homozygotes and a deficit of Arg/Pro heterozygotes. Our present data suggest that the p53 polymorphism affects the risk of lung cancer unrelated to smoking.