Nitric oxide (NO) and its derivatives can directly cause DNA damage and mutation in vitro and may play a role in the multistage carcinogenic process. It has been reported that NO induces mutation in the p53 tumor suppressor gene; we therefore analyzed the relationship between NO synthase (NOS) activity and p53 gene status in early-stage lung adenocarcinoma. Surgical samples were classified into two categories: 14 lung adenocarcinomas with high NOS activity (>25 pmol/min/g tissue, category A), and 16 with low NOS activity (<25 pmol/min/g tissue, category B). A yeast functional assay for p53 mutations disclosed a red colony that corresponded to a mutation in the p53 gene in 8 cases (57.1%) in category A and 3 cases (18.8%) in category B, the frequency being significantly higher in the former (P<0.05). A p53 DNA sequence analysis revealed that 5 of the 8 p53 mutation-positive samples in category A had a G:C-to-T:A transversion, which is reported to be a major target of NO. The mechanism of carcinogenesis of adenocarcinoma is not fully understood, but these results suggest that an excess of endogenously formed NO may induce a p53 gene mutation containing mainly G:C-to-T:A transversion in the early stage of lung adenocarcinoma. Our results suggest that NO has potential mutagenic and carcinogenic activity, and may play important roles in human lung adenocarcinoma.