We describe 19 cases of pulmonary disease due to Mycobacterium xenopi, a nosocomial infection related to the hospital water system. Pre-existing lung disease and prolonged environmental exposure during previous hospitalizations were apparent predisposing factors. Twelve patients had respiratory symptoms, including three with hemoptysis, at the time an abnormal chest roentgenogram was obtained. The predominant radiographic presentation of lung diseases caused by M. xenopi was a nodular or mass shadow, but cavitary disease and multiple nodular densities were also frequently observed. One subject had a solitary pulmonary nodule, and surgical resection was performed. In 12 patients who were skin tested with both M. xenopi sensitin and PPD-tuberculin, induration was consistently greater with M. xenopi. Initial isolates of M. xenopi were uniformly sensitive in vitro to 2.0 microgram of streptomycin, 1.0 microgram of isoniazid, and 10.0 microgram of para-aminosalicylic acid. In general, disease due to M. xenopi was successfully treated with standard antituberculosis drugs.