Several cytokines are involved in the pathophysiology of sarcoidosis. Granulocyte-macrophage colony-stimulating factor (GM-CSF) may be one of these cytokines because its mRNA is expressed by inflammatory cells obtained from the sarcoid lung. We thus asked two questions. Is GM-CSF secreted by bronchoalveolar lavage (BAL) cells? What type of cells express GM-CSF? GM-CSF secreted by the cultured BAL cells from 19 untreated sarcoid patients was measured by an enzyme-linked immunosorbent assay. The individual cells expressing GM-CSF mRNA were identified by in situ hybridization in seven patients. Spontaneous release of GM-CSF by BAL cells was demonstrated in one patient. When stimulated, BAL cells secreted GM-CSF in all subjects studied. In situ hybridization with digoxigenin-labelled GM-CSF cRNA probe revealed that the positive cells were round in shape and larger than lymphocytes, but smaller than large alveolar macrophages that are often irregular in shape. These results strongly indicated that small alveolar macrophages secrete GM-CSF in the sarcoid lesion. This phenomenon may contribute to the enhanced immunological activities observed in sarcoidosis.