Type 2 diabetes, obesity, and breast cancer risk among Japanese women of the atomic bomb survivor cohort.

•The role of obesity and diabetes in breast cancer etiology varies across geographic areas and populations of different ethnic background.•The long-term cohort of Japanese atomic bomb survivors has relatively low obesity and diabetes prevalence.•Among this cohort of Japanese women, excess body weight predicted higher breast cancer incidence while a history of diabetes did not.•Despite a lower BMI in this Japanese population than in Western countries, women with excess weight had a higher risk of developing breast cancer.