Chronic pulmonary histoplasmosis is best regarded as an opportunist or saprophytic infection of abnormal pulmonary spaces by a fungus of very low human pathogenicity. Tissue disease results from host immune response to dispersions of soluble antigen from these focal sources. There are two distinct types of clinical and radiological response. One is an acute or subacute illness manifested by often large segmental pneumonic lesions which tend to heal and are designated as early lesions. The other, usually developing as a complication of the first, is a chronic disease marked by persistent cavitation, low gard chronic illness, and a tendency to promote pulmonary fibrosis and often progressive pulmonary insufficiency. The early lesion is a segmental interstitial pneumonitis with central areas of infarct-like necrosis often adjacent to bullous disease and often outlining prominent emphysematous spaces which appear as radiolucencies. These radiological findings are further characterized by early clearing of the interstitial components, infarct-like contraction of the necrotic zones, obliteration of much of the contained emphysematous and bullous spaces, and healing attended by considerable loss of lung volume. Symptoms are variable but tend to be mild. Malaise, fatigability, low-grade fever, aching chest pain and mild cough lasting a few days to a few weeks are usual. Symptoms are ameliorated by rest. Rest and diminished activity are recommended as treatment. Under these circumstances, 80% of early lesions heal completely and probably most of these would heal spontaneously. Any subsequent course of the disease depends on whether or nor large air spaces, adjacent to or contained within the area of pneumonitis, become infected and persist as cavities. This occurs in 20% of early lesions. Once established, an infected cavity tends to persist and to be attended by symptoms of chronic bronchitis with chronic cough and sputum, fatigability, anorexia, and weight loss. Persisting thickwalled cavities often induce gradual development of pulmonary fibrosis, particulary in the lung bases, apparently from aspiration of antigenic material. This and the accelerated obstructive bronchopulmonary disease often lead to progressive pulmonary insufficiency. The use of amphotericin B is recommended for all persistent thick-walled cavities and in some circumstances surgical resection may be indicated.