From 1982 to 1988, 20 patients with pulmonary nocardiosis were diagnosed at the Department of Medicine, Chulalongkorn Hospital University. The infection was found to be common in immuno-compromised hosts particularly in patients who were suffering from lymphoreticular malignancy, systemic lupus erythematosus, nephrotic syndrome, pulmonary alveolar proteinosis and in patients who were receiving corticosteroids. The clinical manifestations were usually nonspecific. Diagnosis of pulmonary nocardiosis in cases who presented with a short duration of fever and productive cough was often delayed because they were considered to have acute bacterial pneumonia. The findings on chest roentgenogram were nonspecific as nonhomogeneous airspace infiltrates, cavitary lesions, nodule, or miliary infiltrates. The complete blood count frequently showed leukocytosis and neutrophilia. The diagnosis of nocardiosis was suspected if the staining of specimens obtained from the lesions showed typically weakly gram-positive and modified acid-fast branching filament organism and the diagnosis was confirmed by culture. The skin and the central nervous system were the most common hematogenous disseminations. Sulfamethoxazole and trimethoprim in combination were the drugs of choice. The treatment for a minimum of 6 months was appropriate in order to prevent relapse. Poor prognostic factors in nocardiosis were acute infection, Cushing's disease; and disseminated infection involving the central nervous system.