BACKGROUND: Adiposity is associated with increased risk of breast cancer in post-menopausal women, while the association appears to be the inverse among premenopausal women in Western populations, but is unclear for Chinese women. METHODS: Using data on 284 538 women with no history of cancer, hysterectomy, oophorectomy, or breast surgery at baseline in 2004-8 from the China Kadoorie Biobank (a prospective cohort study), Cox regression was used to estimate adjusted hazard ratios (HRs) for breast cancer and its subtypes by measured body mass index (BMI) and other adiposity measures (waist circumference, fat percentage, waist-hip ratio, fat mass) and by self-reported BMI at age 25 years. RESULTS: Mean BMI was 23.8 (3.5 SD) kg/m2 at baseline (mean age 51.4 years) and 21.9 (2.7 SD) kg/m2 at age 25 years. During 10 years of follow-up, there were 2379 incident breast cancer cases. Higher BMI was associated with a higher risk of breast cancer in women who were post-menopausal [HR = 1.35, 95% confidence interval (CI) 1.24-1.47] and premenopausal (HR = 1.13, 95% CI 1.03-1.24) at baseline, but not when censoring at age 50 years to capture mainly premenopausal cancers. Among post-menopausal women, BMI was associated with oestrogen-receptor positive (ER ) (HR = 1.47, 95% CI 1.24-1.74), but not with oestrogen-receptor negative (ER-) breast cancer (HR = 1.02, 95% CI 0.75-1.37). Waist circumference and fat percentage were associated with a higher risk of breast cancer. CONCLUSION: In this cohort of Chinese women, higher levels of general and central adiposity were associated with a higher risk of breast cancer, in both women who were premenopausal and women who were post-menopausal at baseline. Among post-menopausal women, adiposity was associated with ER   but not ER - breast cancer.