Abnormalities of the p53 oncogene in lung cancer have recently been reported to be more frequent in small cell lung cancer (SCLC) than in non-small cell lung cancer (non-SCLC), but their status in combined type SCLC is as yet unknown. In this study, immunohistochemical analysis using a polyclonal antibody against p53 protein was performed in 12 surgically resected specimens of combined type SCLC. Immunoreactivity of the p53 protein was found in 5 (42%) of the 12 cases, and the immunostaining pattern of the p53 protein in areas of the non-small cell carcinoma type was the same as in those of the small cell carcinoma type. Thus, it seems that the incidence of p53 abnormalities in combined type SCLC is slightly lower than in ordinary type SCLC. It is also suggested that abnormalities of the p53 oncogene in this histological type may not be a specific event related to the morphological difference between small cell carcinoma and non-small cell carcinoma.