Mycobacterial disease is common among patients with the acquired immunodeficiency syndrome (AIDS) and human immunodeficiency virus (HIV) infection. Among all patients with AIDS, the most frequently isolated cause is Mycobacterium avium complex; but in some groups, such as Haitians and intravenous drug users, M. tuberculosis is commoner. Extrapulmonary disease and noncavitary, nonapical pulmonary tuberculosis are frequently seen. It is recommended that initial treatment of tuberculosis in these patients include at least three standard antituberculosis drugs and that therapy be continued for a minimum of 9 months. Treatment of disseminated disease due to M. avium complex with this regimen is unsatisfactory, and a four-drug regimen that includes two experimental drugs, rifabutine and clofazimine, is the recommended treatment. Patients with AIDS or HIV infection and pulmonary tuberculosis should be considered potentially infectious, and appropriate infection-control and contact-tracing procedures applied. Persons with HIV infection should be given a tuberculin skin test and, if the reaction is positive (10 mm or more in diameter), isoniazid preventive therapy.