Fifty-five patients with Mycobacterium kansasii isolates (47 pulmonary and eight disseminated) were identified at a large Texas hospital from 1975 to 1985. The mean age of patients was 60 years, and there was a slight male predominance. Isolation of M. kansasii usually represented disease. The great majority of patients with pulmonary infection due to M. kansasii had underlying pulmonary diseases, and 70% had nonpulmonary predisposing factors. M. kansasii pulmonary disease clinically and radiographically resembled pulmonary tuberculosis. Disseminated M. kansasii infection occurred in severely immunocompromised patients, who frequently had pulmonary predispositions as well. Disseminated infection most of ten involved the lung, reticuloendothelial system, bone, joint, and skin and presented with signs and symptoms related to these organs. Despite only moderate in vitro susceptibility of M. kansasii to routine antituberculous drugs, most patients responded to rifampin-containing regimens. The prognosis of patients with M. kansasii disease was determined primarily by their underlying diseases.