Lung cancer represents one of the major human carcinomas with the highest degree of mortality. Epidemiologic studies have linked this disease to "chronic injury," largely induced by cigarette smoking. In the present studies, we demonstrate the in vivo expression of platelet-derived growth factor (PDGF) and PDGF receptor (PDGF-R) beta mRNAs and their respective protein products in malignant epithelial cells of primary human lung carcinomas. In contrast, nonmalignant epithelial cells in control, normal lung tissue specimen did not express PDGF and PDGF-R mRNAs and did not produce their respective protein products. Epithelial cells in lung specimen from patients with idiopathic pulmonary fibrosis expressed only PDGF mRNA but not PDGF-R beta mRNA. These findings of the inappropriate coexpression of a potent mitogen, PDGF, and its receptor in lung cancer epithelial cells suggest the presence of a powerful in vivo mechanism contributing to the self-stimulation and unregulated growth of lung cancer tumor cells.