As the prevalence of tuberculosis has decreased, diseases due to nontuberculous mycobacteria have assumed a greater importance, and the common occurrence of disseminated M. avium complex disease in AIDS patients has stimulated interest in these organisms. Skin test surveys indicate that infection by these mycobacteria is common, but disease is infrequent, with an estimated prevalence in the United States of approximately 2 per 100,000. The most common forms of disease are chronic pulmonary disease resembling tuberculosis, benign cervical adenopathy in children, skin and soft-tissue infection, and disseminated disease in immunocompromised persons. Recent studies have increased our understanding of the ecology of these organisms, which are found in water, soil, and aerosols. However, much remains to be learned about transmission of infection and pathogenesis of disease. Increased understanding in these areas will be important in the prevention of nosocomially acquired disease and disseminated disease in immunocompromised patients.