Several analyses of the retinoblastoma (RB) gene in lung cancer at the DNA, mRNA and protein levels have recently been reported. In particular, small cell lung carcinoma shows a high incidence of RB gene abnormalities, suggesting that alterations of this gene may participate in tumor development. In the present study, we used an immunohistochemical technique with a monoclonal antibody raised against RB protein (PMG3-245) to detect its expression in representative paraffin sections of tissues obtained from 108 patients with various types of lung cancer treated by surgical resection of the primary tumor. While deletion of RB protein expression was observed in 7 (88%) of small cell lung carcinomas, only 17 (17%) of 100 non-small cell lung carcinomas showed decreased RB protein levels and 6 (6%) showed no RB protein expression. This low incidence of RB protein expression abnormalities in non-small cell lung carcinomas was significant (p < 0.0001). Thus, in contrast to small cell lung carcinoma, abnormalities in RB protein expression may be minor events in non-small cell lung carcinoma. In addition, no significant correlation was found between abnormalities in RB protein expression and clinical factors such as stage, tumor size, and nodal involvement in non-small cell lung carcinoma. However, abnormalities in RB protein expression in squamous cell carcinoma were observed only in the less differentiated types (p = 0.144), and there was a weak but not statistically significant association in non-small cell lung carcinoma between RB protein status and prognosis (p = 0.09). Therefore, in non-small cell lung carcinoma, although abnormalities in RB protein appear not to be closely associated with tumor development, further studies on a larger scale and with a longer-term follow-up are required to determine the clinicopathological significance of RB gene abnormalities, in particular the relationship between abnormalities of RB protein and differentiation or prognosis.