Tuberculosis of the lungs is the most important and most insidious of the relapsing infections. Often without symptoms or signs of activity for years, it characteristically surprises patient and physician by exploding without warning from so-called "stability" into grossly progressive disease. It may then become quiescent again, only to repeat the cycle months or years later. Sometimes the process is repeated several times, to terminate fatally decades after the first episode. Thousands of patients have thus died of small improvements followed by fresh bronchogenic disseminations which could not be anticipated. It has always been true that some patients recover and remain well. This was the case prior to chemotherapy. The incidence of relapse, however, in the prechemotherapy era, ranged from 24% to 56% in treated cases.* It has become clear in the last several years that chemotherapy has an important role in the treatment of all cases of active tuberculosis.