Physiologic data concerning 30 patients with bronchogenic carcinoma were obtained before radiation therapy and at intervals thereafter. At pretreatment examination, abnormalities were found in all patients. Postirradiation data revealed decreased residual volumes and diffusing capacities with little progression of airway obstruction. No parameter evaluated demonstrated correlation with the volume dose of the tissue irradiated. Pretreatment lung scintiscans disclosed areas of decreased capillary blood flow which were greater at times than would be suggested by chest roentgenograms. Postirradiation lung scans showed further decreases in the pulmonary capillary blood flow in areas initially affected. It is suggested that alterations in the capillary blood flow and resultant tissue changes were initiated by tumor and aggravated by radiation therapy. The data do not confirm other reports attributing the development of severe pulmonary disability to radiation therapy of the lung.