p53 is a tumor suppressor gene, located in the short arm of chromosome 17, which encodes for a nuclear protein involved in the control of cellular growth. Mutations in p53 gene are the most common genetic alterations in a several human cancers, including Non Small Cell Lung Cancer (NSCLC). However, up to now, the role of p53 in the tumour's behaviour and its progression has not been completely clear. We performed immunohistochemical staining for mutated p53 using two monoclonal antibodies, PAb1801 and PAb240, in fresh tumour specimens from 103 consecutive patients who underwent surgery for resectable NSCLC. PAb1801 detects both the normal and mutant form of p53, while PAb240 is specific only for the mutant form and recognizes a denaturation-resistant epitope located between aminoacids 156-335. Both antibodies showed a mainly nuclear staining in neoplastic cells but not in surrounding uninvolved lung tissues. 68 out of 100 (68%) and 37 out of 103 (35.9%) of the cases were positive with PAb1801 and with PAb240, respectively. Tumours from patients with hilar-mediastinal lymph node involvement showed a higher p53 expression, detected by PAb1801, than those without nodal metastases (p = 0.04). Moreover, tumours expressing more than 60% of positive cells with both antibodies showed a significant increase of nodal involvement (p = 0.1; p = 0.03). Furthermore, p53 expression was significantly related to post-surgical stage (p Tumor Stage) (p = 0.04). In addition, we did not find any correlation between p53 expression and proliferating activity evaluated by PCNA, Ki-67 and DNA flow cytometric cell cycle. In conclusion, the evaluation of p53 oncogene expression may identify individuals whose resectable NSCLCs have a more aggressive tumour behaviour.