Chen Y, et al. - Here, the authors investigated the different effects of body mass index (BMI) on the risk of breast cancer among premenopausal and postmenopausal women, and examined the potential factors that influence the associations. Different effects of BMI was observed on pre-menopausal and postmenopausal breast cancer risk, similar to previous studies. In contrast, a high BMI was not correlated with decreased risk in total pre-menopausal women.