We described five patients in whom pulmonary malignant histiocytosis was histologically confirmed during life. Pulmonary symptoms dominated the clinical presentation in three patients, and one patient had a 5-year history of pulmonary malignant histiocytosis controlled by steroids. Radiologic features were nonspecific. Most patients had bilateral reticulonodular or fluffy infiltrates. Recurrent pulmonary histiocytosis in one patient was manifest by bilateral pulmonary nodules. Pathologically, pulmonary infiltration by malignant histiocytosis followed the normal lymphatic pathways of the lung along bronchovascular rays, in interlobular septa, and within the pleura. Unusual features included marked septal edema and fibrosis out of proportion to the degree of infiltration, and in one case, the marked predilection fo the infiltrate to occlude small airways ("malignant histiocytosis bronchiolitis"). Three patients had microscopic nondestructive nodules adjacent to lymphatics. A pulmonary recurrent in one patient was composed of large monomorphous nodules with central necrosis and prominent vascular infiltration by malignant cells. The pulmonary infiltrate of malignant histiocytosis was often heterogeneous, and included variable numbers of lymphocytes and plasma cells intermingled with alveolar macrophages and metaplastic alveolar lining cells. The cytologic features of the infiltrate varied from benign to pleomorphic and obviously malignant. Histologic features which may obscure the correct diagnosis, as they did initially in three of our cases, include: malignant histiocytic bronchiolitis, marked septal edema and fibrosis; the heterogeneous cellular infiltrate; and in one case, benign cytologic features. The most valuable initial clue to the correct diagnosis was the tendency of the infiltrates to follow lymphatics of the lung.