The incidence of tuberculosis in the United States declined steadily until 1985, while at the same time, for at least the past 15 years, the frequency of disease attributable to other mycobacteria increased both in actual numbers and in the proportion of the total burden of mycobacterioses. Chronic pulmonary disease, lymphadenitis in children, skin and soft-tissue involvement, and infections of the skeletal system were predominant, and the principal etiologic agents were Mycobacterium avium/Mycobacterium intracellulare complex. Mycobacterium kansasii, Mycobacterium marinum, Mycobacterium fortuitum/Mycobacterium chelonae complex, and Mycobacterium scrofulaceum. Since 1986 disseminated disease has become not only more common, especially in association with opportunistic infections in patients with AIDS, but also attributable in part to the growing population of patients who are immunocompromised because of malignancy, receipt of an organ transplant, and administration of steroids. Treatment of these patients has been difficult because of the frequency of severe underlying conditions and the natural resistance of most of the nontuberculous mycobacteria to the presently available drugs.