An unusual case of Mycobacterium avium complex infection occurred in a young adult with no preexisting disease and no evidence of immunodeficiency. There was diffuse interstitial involvement of the lungs which suggested an active alveolitis. Diagnosis required open-lung biopsy. Restriction fragment length polymorphism analysis and multilocus enzyme electrophoresis indicated that the source of the infection was a hot tub. The infection proved to be exceptionally responsive to treatment, and there was complete resolution with a four-drug regimen.