In 67 patients with chronic pulmonary tuberculosis, regional pulmonary arterial blood flow was measured by quantitative radioisotope scanning of the lungs after intravenous administration of macroaggre-gated iodinated I131 serum albumin. The most significant finding was a decrease in the pulmonary arterial blood flow to the diseased areas in all cases, usually to a greater extent than had been expected from the size of the lesion in the chest roentgenogram. Lung scanning was helpful in the evaluation of the relative involvement of the two lungs, and in several cases aided in the decision regarding the extend of pulmonary resection. Because of its safety and technical simplicity, lung scanning was helpful in serial quantitative studies of the regional pulmonary blood flow to the lungs in pulmonary tuberculosis.