The diagnostic significance of p53 and bcl-2 proteins in epithelial non-small-cell lung cancers was examined, and the relationship between these proteins expression and other disease parameters, including stage of the disease and tumor differentiation, were studied. We analyzed p53 and bcl-2 proteins expression in 60 imprint smears of freshly resected lung tumors (37 squamous and 23 adenocarcinomas) using the immunocytochemical technique. There were seven patients with stage I disease, 24 with stage II, 23 with stage IIIa, and six with stage IIIb disease, according to the International Staging System classification. Sixteen of the tumors were bcl-2 positive and 25 were p53 positive. Twenty tumors were negative for both bcl-2 and p53 (33.3%). Statistical analysis showed no association between the incidence of p53 or bcl-2 positivity. Adenocarcinoma or squamous carcinoma analysis showed significant associations between p53 positivity and poor differentiation and advanced disease stage as well as bcl-2 and early disease stage and well-differentiated tumors. There was also an association between the stage of the disease and the degree of differentiation of the tumors. In conclusion, bcl-2 positivity must be considered a good prognostic sign. On the other hand, p53 positivity seems to indicate, even in tumors at a relatively early stage, that a serious aggressive tumor which will not be easily eradicated is present.