Although the clinical effects of successful tuberculosis chemotherapy are obviously beneficial and dramatic, the anatomic and histologic changes are inconspicuous and subtle. They consist, for the most part, in the absence of those lesions and reactions found in patients dying of pulmonary tuberculosis. It is the purpose of this report to compare the findings in tissue surgically resected for pulmonary tuberculosis in a group of patients who did receive chemotherapy with a group of patients who did not receive chemotherapy. Feldman and Hinshaw (1) first reported the lack of lesions or the presence of a few abortive tubercles in guinea pigs treated with streptomycin after being injected with virulent tubercle bacilli. Mahon (2) stated that with streptomycin therapy, miliary tubercles healed more rapidly, cavities contained unusually vascular granulation tissue, and bronchi were surprisingly free of tubercles; however he also noted that fundamental tissue reactions were not altered by the treatment. Koyama (3) reported that chemotherapy prevented spread of the disease and prevented cavitation and empyema. Auerbach (4) believed that chemotherapy produced specific qualitative and quantitative differences in tissue reaction to infection, which included a rapid and extensive decrease in perifocal reaction, a decrease in the width of the capsule and thickness of cavity wall, a decrease in pulmonary fibrosis, emphysema, aneurysms and hemorrhage, and, frequently, an unusual mode of cavity healing (open healing). Laennec (5) had described open healing and clean cavities and, though not unknown since then, it was considered a rare event