The histologic and clinical features of pulmonary histiocytosis X are discussed. The majority of patients are in the third and fourth decades of life. They present with nonspecific respiratory complaints and bilateral reticulonodular infiltrates that can be seen on chest radiographs. The histologic lesions of pulmonary histiocytosis X are scattered, discrete nodules that frequently center on small airways. The initially cellular nodules are replaced progressively by fibrous tissue. The cell infiltrates in active lesions contain variable numbers of eosinophils and histiocytes resembling Langerhans cells of the epidermis (Hx cells). In most patients the disease resolves or stabilizes, leaving few, if any, significant residues. A minority of patients develop progressive pulmonary disease that is ultimately fatal. The cause of pulmonary histiocytosis X is unknown, but two recent series suggest a relationship to cigarette smoking.