Langerhans' cells were found in lung biopsies in one of nine control patients and in 20 of 160 patients with fibrotic lung disorders, including 13 of 56 patients with idiopathic pulmonary fibrosis, two of nine patients with collagen vascular diseases, two of seven patients with hypersensitivity pneumonitis, and each of three patients with end stage fibrosis of uncertain cause. Langerhans' cells were not found in any of the 41 patients with sarcoidosis, the 35 patients with interstitial lung diseases associated with inhalation of inorganic dusts, the seven patients with pulmonary lymphangioleiomyomatosis, or the two patients with chronic eosinophilic pneumonia. In the control patient, Langerhans' cells were found between epithelial cells in bronchioles. In patients with fibrotic lung disorders, Langerhans' cells were found in the epithelial layer of bronchioles and alveoli containing proliferating epithelial cells, i.e., either cuboidal epithelial cells of bronchiolar origin or type II alveolar epithelial cells. Severe fibrosis or squamous metaplasia were not prerequisites for the presence of Langerhans' cells. The motility of Langerhans' cells apparently was restricted, as they were not found in the air spaces in any of the biopsies, and they were not recovered from bronchoalveolar lavage fluid of any of the 97 patients studied, even though some of these patients had relatively numerous Langerhans' cells in lung biopsies. These observations are in sharp contrast to those in pulmonary histiocytosis X, in which histiocytosis X cells (HX cells) occur in granulomas, in alveolar interstitium, and between epithelial cells of the lower respiratory system. HX cells also migrate into air spaces, as shown by their occurrence in bronchoalveolar lavage fluid. The HX bodies in HX cells are morphologically similar to Langerhans' cell granules, but are more numerous and pleomorphic. HX cells are considered to be reactive or activated Langerhans' cells.