p53 abnormalities appear to play critical roles in human carcinogenesis. Many studies have indicated that p53 alteration is responsible for the differences in clinical characteristics of non-small cell lung cancer (NSCLC). The purpose of this review is to evaluate and summarize the significance of p53 alteration in the diagnosis and therapy of NSCLC. After assessing the principles of each approach to detecting p53 alteration, we reviewed the literature describing the impact of p53 alteration on clinical modalities such as diagnosis, prediction of prognosis, and response to treatment. We also reviewed p53 gene replacement therapy for NSCLC. Although p53 appears to be the most important gene in the development of NSCLC, it is not known whether p53 abnormalities are useful markers for clinicians. A multicenter, prospective study of p53 immunohistochemistry should therefore be performed. Replacement of the p53 gene has been shown to be effective in a clinical trial for patients with NSCLC. In addition to simple gene replacement, combination treatments of gene transfer and chemotherapy are now being tested in clinical trials.