Tuberculosis is not only treatable but preventable as well. A number of options exist for reducing the transmission of tubercle bacilli. Practitioners are in a position to intervene directly on behalf of their patients and indirectly by promptly reporting infectious patients to local public health authorities. A considerable body of information supports the use of isoniazid in persons who are infected by Mycobacterium tuberculosis and who also have readily identifiable characteristics that place them at increased risk for developing tuberculosis.