The purpose of this study was to determine the prognostic significance of the expression of p53 and Ki-67 in non-small cell lung cancer (NSCLC) using immunocytochemical detection. All consecutive NSCLC cases were selected for study, and, after surgery, a part of each tumor sample was frozen at -20 degrees C and stored for immunocytochemical studies. Overexpression of p53 was associated significantly with worse patient outcome in stage I disease, whereas no excess risk was evident in stage II and III cases. The same pattern was observed for Ki-67 expression. The excess risk in stage I cases with p53 and Ki-67 overexpression was observed only in adenocarcinoma. These findings are in agreement with other retrospective studies and support the hypothesis that p53 alteration may have different roles in adenocarcinoma and in squamous cell carcinoma, such as a carcinogenic factor for both cellular types but progression only for adenocarcinoma.