Bcl-2 oncoprotein, a member of a new category of oncogenes associated with the regulation of programmed cell death (apoptosis), has been considered to be involved in biological processes such as tumorigenesis and tumor development. To determine the role of bcl-2 oncoprotein in lung cancer, we preliminarily examined the expression of this protein in various histological types. Immunohistochemical staining using monoclonal bcl-2 oncoprotein antibody was performed in surgically resected frozen specimens. Bcl-2 staining was seen in nine of 13 small cell lung cancers (69%), while only 18 out of 69 non-small cell lung cancers (26%) expressed bcl-2 oncoprotein, showing a significantly increased incidence of bcl-2 oncoprotein expression in the former histological type. Considering the greater aggressiveness of small cell lung cancer compared to non-small cell lung cancer, the possibility exists that the high prevalence of bcl-2 oncoprotein expression in small cell lung cancer is closely associated with tumorigenesis and tumor development.