Although MDM2, p21/WAF1, and p53 are considered as regulating each other based on in vitro studies, the relation in human lung cancer is not fully understood. The expressions of these proteins were examined immunohistochemically in 112 resected non-small cell lung cancer specimens and the correlation between them were analyzed. MDM2 was expressed in 45% of all lung cancers. In advanced stage, MDM2-positive cases were observed more frequently than in early stage, showing significant difference. No significant difference was observed in the prognosis of the patients regardless of the expression of any protein. Although no correlation was observed between MDM2 expression and p53 expression, or between p21/WAF1 expression and p53 expression, MDM2 expression was strongly related with p21/WAF1 expression. Therefore, MDM2 expression may relate to the progress of the stage of lung cancer, and MDM2 expression and p21/WAF1 expression may be associated not through the p53-related pathway.