Retinoblastoma (RB) protein expression was examined in paraffin and frozen tissue sections of 36 primary non-small cell lung carcinomas (NSCLC) using immunohistochemistry with confirmation by direct Western blotting. A normal RB protein staining pattern was present in 24 and absent in 10 NSCLC. Two additional RB positive primary tumors have major foci in which all tumor cells showed no RB protein staining. Significantly more high-stage (stages III and IV) NSCLC had altered RB protein expression than those with low-stage (stages I and II) tumors (P less than 0.05). The results suggest that absence of the RB expression may be associated with the initiation and/or progression of many NSCLC. This is the first report of successful RB staining in paraffin sections.