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Background: It has been suggested that the association between body mass index and breast cancer risk differs between Asian women and Western women. We aimed to assess the associations between body mass index and breast cancer incidence in East Asian women., Methods: Pooled analyses were performed using individual participant data of 319,189 women from 13 cohort studies in Japan, Korea, and China. Participants' height and weight were obtained by measurement or self-reports at cohort baseline. Breast cancer was defined as code C50.0-C50.9 according to the International Classification. Using a Cox proportional hazards model, hazard ratios of breast cancer were estimated for each body mass index category, with the reference group set as the group with a body mass index of 21 to < 23 kg/m 2 . The hazard ratio for a 5 kg/m 2 increase in body mass index was also calculated., Results: During a mean 16.6 years of follow-up, 4819 women developed breast cancer. Similar to Westerners, a steady increase in breast cancer risk with increasing body mass index was observed in postmenopausal women, but the slope of the risk increase appeared to slow at a body mass index of 26-28 kg/m 2 . In premenopausal women, the inverse association seen in Westerners was not observed. The risk of developing breast cancer after 50 years of age increased slightly with increasing body mass index, which was more pronounced in the older birth cohort. There was no significant association between body mass index and the risk of developing breast cancer before 50 years of age, but the risk estimates changed from positive to negative as the birth cohort got younger., Conclusions: In East Asia, the role of body mass index in breast cancer in premenopausal women may be changing along with the increase in obesity and breast cancer. The increased risk of postmenopausal breast cancer with a higher body mass index was as robust as that of Western women., Competing Interests: Declarations Ethics approval and consent to participate This study was performed in accordance with the Declaration of Helsinki. Each participating study was approved by the relevant institutional ethical review boards. The protocol for the ACC analysis was approved by the institutional review board of the National Cancer Center, Japan. The requirement for consent from several cohort studies was waived due to the justification for lack of consent at the time of the survey, the difficulty in obtaining new consent, and the public necessity of the research. Consent for publication All authors reviewed and approved the final version for submission. Competing interests The authors declare no competing interests., (© 2024. The Author(s).)