Mutations that prevent the normal expression of the retinoblastoma susceptibility gene (RB) have been linked to the pathogenesis of several human malignancies. Mutational inactivation of this tumor-suppressor gene appears to initiate the development of retinoblastoma, and may also contribute to the pathogenesis of osteosarcomas, soft-tissue sarcomas, small-cell lung cancer and other malignancies. In cooperation with the Lung Cancer Study Group, we studied the structure and expression of the RB gene in a cohort of 219 primary non-small-cell lung cancers (NSCLCs). RB gene structure was studied at the DNA level by Southern blot and chromosome 13 restriction fragment length polymorphism analyses. Expression of the RB gene was evaluated by Northern analysis of the transcript and immunohistochemical analysis of the protein (p105RB). Immunohistochemistry of the RB protein proved to be the most sensitive method for the detection of Rb gene inactivation. Absent or abnormal RB protein staining was detected in 53/163 (32%) evaluable cases studied, while Northern analysis showed 22/219 cases to have an altered or absent RB transcript. Southern analysis revealed only two cases of structural alteration of the gene, but loss of heterozygosity from chromosome 13 was common in tumors that failed to express the protein. Analysis of the clinical outcomes of the patients whose tumors were studied did not show any correlation of RB inactivation with time to relapse or death. The data from this study indicate that the RB gene is inactivated in a significant number of NSCLCs. The role that these mutations may play in the development of NSCLC remains to be defined.