67 patients with tumors 3 cm or less and with negative axilla, who had lumpectomy and radiotherapy, and 122 patients with larger tumors or clinically positive axilla, who had radical radiotherapy without prior lumpectomy, were evaluated at five years to determine the optimum dose for local control. 7000 rad in seven to eight weeks controlled 85% of lumpectomy cases, whereas 8000 rad in 81/2 weeks were required to control two-thirds of cases treated by radical irradiation alone. Subclinical (N0, N1a) and clinical disease (N1b) in the axilla was controlled, in a very high percentage, when 6000 and 7000 rad were delivered, respectively. Dose response curves were obtained for clinical disease in the breast and axilla. No such response was elicited for subclinical disease. Undifferentiated cancers had higher recurrence rates than well differentiated tumors. Radiation fibrosis in lumpectomy cases was insignificant, whereas 10% of radically irradiated patients had fibrosis of the breast.