This is the final report of 30 years of observation of isoniazid prophylaxis and chemotherapy of tuberculosis in children. It includes 2,494 patients who completed a course of prescribed therapy in Houston and who had been under observation for a total of 15,943 person-years. The effectiveness of isoniazid prophylaxis was best demonstrated in those children infected before 4 years of age, because none of them have experienced overt disease. Chemotherapy of pulmonary tuberculosis was effective for existing disease in all ages and prevented dissemination. Of those treated for pulmonary tuberculosis during childhood, the striking absence of adolescent reactivation points to the likelihood of a permanent cure. Adequate drug therapy is the key to successful treatment.