Three hundred and sixty cases of breast cancer were collected from among the 63,000 female members of the Radiation Effect Research Foundation (RERF) extended Life Span Study sample which includes atomic bomb exposed women and controls of Hiroshima and Nagasaki. The relationship of these breast cancer cases to A-bomb radiation was sought, and in studying of 5-year survival, the following conclusions were obtained concerning its relationship to histopathological findings. The prognosis of the 50+ rad high dose group is the best, followed by the nonexposed group and the low dose group. The apparently better survival may be due to the fact that this group is heavily weighted in favor of those who were younger at the time of the bomb (ATB). There is no specificity of the histologic type of breast cancer in the survivors by dose. Nor is any significant difference observed in the distribution of tumor size and histological grade, though the survival rates of this series are worse than those of usual Japanese women with breast cancer. Cellular reaction is significantly marked in the stroma of carcinoma tissue in the high dose group. Immune reaction is considered to be strong in the affected site of breast cancer in the high dose group and this can be regarded as a finding suggestive of good prognosis. Further extended studies are therefore warranted.