Regional and total lung function measurements and chest radiographs were obtained from 18 patients with cancer of the breast from 3 months to 6.7 years after the start of treatment. A control group of 20 patients was studied before radiotherapy but after mastectomy. The functional parameter most affected by radiation was blood flow. In some cases in which the radiographic changes were mild the functional measurements indicated severe vascular damage. The radiation appears to reduce the number and efficiency of functioning lung units within the irradiated region.