Mutations in the p53 gene are common in many cancers. Nevertheless, the relationship between mutations of this tumor suppressor gene and patient survival in non-small cell lung cancer (NSCLC) remains unclear. Interpretation of prior studies of patient outcomes are complicated by the inclusion of both surgical and nonsurgical patients. To better isolate the potential effects of p53 gene mutations per se on tumor progression, we chose to examine patients with advanced disease in whom surgery was not performed (stages IIIA, IIIB, and IV). We have used PCR-denaturing gradient gel electrophoresis, a sensitive and specific method for the detection of a variety of p53 mutations in cytology or biopsy specimens, to evaluate the prognostic significance of p53 gene mutations in nonsurgical patients with advanced NSCLC. In 70 consecutive medical patients, p53 mutations were found in 29 cases (41%) at the time of initial diagnosis. Followed prospectively, patients with p53 mutations had a significantly reduced survival time after diagnosis than those without mutations (median survival, 17 versus 39 weeks; P = 0.0003) independent of other clinical factors. This abbreviated survival occurred in both patients who received chemotherapy (n = 39, P = 0.002) or best supportive care (n = 31, P = 0.018). These results indicate that mutations of the p53 gene in patients with NSCLC who do not undergo surgical resection portends a significantly worse prognosis.