Forty-two patients had radiotherapy to the lung for bronchogenic carcinoma. All had pulmonary function studies before therapy and 33 had repeat studies after therapy. In 25 of those who died, autopsy was permitted and the lungs were inflated and formalin fume fixed. Pathologic evidence of radiation injury in bronchi was present in 73 per cent (8 of 11 cases) compared to 40 per cent in lung parenchyma (8 of 20 cases). Roentgenographically, changes secondary to radiation were noted in 29 per cent (9 of 31 cases). In general, these changes as well as the changes in pulmonary function were mild. The diffusing capacity for CO (DLCO) decreased after radiotherapy in 20 of 27 cases. Pulmonary resistance decreased and forced flow rates (FEV1) and residual volume (RV) increased after therapy. These latter changes in function studies were significantly greater in patients with marked obstruction (FEV1 50 per cent N or less) as compared to those without obstruction (FEV1 70 per cent N or more). Patients with chronic obstructive pulmonary disease tolerated radiotherapy as well as patients without evidence of this disease.