Pulmonary Langerhans cell histiocytosis (LCH) is an uncommon disorder occurring most often in young smokers. Histologically, the disease is characterized by granulomatous lesions containing LC that destroy distal bronchioles. The etiology of the disease remains unknown, but progress has been made in understanding its pathogenesis. Modifications in the bronchiolar epithelium induced by smoking, such as the increased secretion of GM-CSF by these cells, are probably responsible for the initial accumulation of large numbers of LC. However, given the rarity of pulmonary LCH compared with the frequency of smoking, an as yet unidentified genetic predisposition may also be necessary for the development of the disease. Although LC in LCH granulomas may be clonal in origin, several observations argue against the idea that the disease, which can regress spontaneously, is a malignant process. Cells of dendritic cell lineage (including LC), are potent antigen presenting cells, suggesting that pulmonary LCH results from an uncontrolled immune response initiated by LC. Consistent with this idea, LC and T-cells are the predominant cell populations found in the early lesions of pulmonary LCH, and unlike LC in the normal bronchial mucosa and those accumulating in other pathologic situations, LC in pulmonary LCH granulomas express surface molecules important for the activation of T-lymphocytes. A number of mediators are produced in the microenvironment of granulomas that probably influence the outcome of the local immune and inflammatory reaction. Ultimately, precise knowledge of the pathogenesis of this disorder should permit the development of specific treatment. In the interim, therapies aimed at modifying the state of activation of LC in the granulomatous lesions may prove useful.