The prognostic significance of nuclear p53 protein expression in survival and local control was investigated immunohistochemically in 36 patients with inoperable or unresectable non-small cell lung cancer who were treated with radiation therapy (RT). Formalin-fixed, paraffin-embedded sections obtained by bronchoscopy were used to examine the expression of nuclear p53 protein with immunohistochemistry. In 25 cases (69%), p53 protein expression was detected. There was no relation between p53 expression and other pretreatment characteristics. Response to RT was found in all p-53 negative cases versus 72% in p53-positive cases (p < 0.05). The 2-year survival rate for p53-negative cases was 51% with a median survival time of 21 months. The corresponding rate for p53-positive cases was 31% with a median survival time of 9 months. This difference, however, did not reach a statistically significant level because of the small sample size. In conclusion, the results of this study suggest that p53 protein expression may be of predictive value on response to RT in non-small cell lung cancer.