Objective: To investigate the association between the risk of breast cancer and body mass index (BMI) andits change in adulthood. Methods: A population-based case-control study was conducted in China from 2008to 2009. The study sample included 643 cases with pathologically-confirmed breast cancer and 590 controls.Information on adult height and weight at diagnosis, at five years before diagnosis, and at age 21 years wascollected by face-to-face interview using a structured questionnaire. Odds ratios (ORs) and 95% confidenceintervals (CIs) were obtained using unconditional logistic regression analyses. Results: There was an increasedrisk of breast cancer associated with overweight or obesity in adulthood in Chinese women. Compared with thewomen who never had overweight or obesity in their adulthood, the adjusted ORs of breast cancer were 1.99(1.42-2.79) for a BMI ≥25.0 at age 21 and just before diagnosis. This rose to 3.04 (1.18-7.86) if, in addition, BMI≥25.0 was also present five years before diagnosis. Conclusion: Weight gain throughout adulthood is associatedwith an increased risk of breast cancer in Chinese women.