G M-CSF has been shown to be important for the survival and function of cells of dendritic cell/Langerhans cell (LC) lineage in vitro. Since these cells have been demonstrated to infiltrate human lung and some lung carcinomas, we hypothesized that the production of GM-CSF in the lung could be important in their recruitment and differentiation. Using both immunohistochemistry and in situ hybridization, we have shown that: (a) GM-CSF was produced by normal bronchiolar epithelium, the only site were CD1a+ LC are observed in the normal lung, whereas neither GM-CSF production nor LC were identified in normal alveolar epithelium. (b) In inflamed pulmonary tissue, hyperplastic alveolar cells produced GM-CSF, and CD1a+ LC accumulated adjacent to these cells. (c) Some, but not all, lung carcinomas produced this cytokine, and a close correlation was found between the production of GM-CSF and the number of CD1a+ LC infiltrating these tumors. Since GM-CSF was produced at all sites where CD1a+ LC are known to accumulate, but not at other locations within the lung, these data suggest that the local production of GM-CSF by certain lung cells may play an important role in determining the distribution and differentiated state of dendritic cell/LC in the human lung.