Background: Overweight increases breast cancer risk in population risk postmenopausal women. Data from the National Surgical Adjuvant Breast and Bowel Project (NSABP) P1 and P2 chemoprevention trials indicate premenopausal overweight, but not postmenopausal overweight, increases breast cancer risk in high risk women. We estimated the relationship between overweight and breast cancer risk in the International Breast Cancer Intervention Studies (IBIS) I and II. Methods: The IBIS prevention trials compared tamoxifen (IBIS-I) and anastrozole (IBIS-II) vs. placebo in women at increased risk of breast cancer. Baseline body mass index (BMI) was calculated for premenopausal (n = 3138) and postmenopausal (n = 3733) women in IBIS-I and postmenopausal women in IBIS-II (n = 3783). BMI change ( < 5%, 5-9.9%, > 10%) was available for 2504 IBIS-I women. There were 542 (IBIS-I) and 168 (IBIS-II) invasive breast cancer events and median follow-up was 19.6 years (IQR 17.8–21.1) and 9.3 years (IQR 7.2–11.4), respectively. We used Cox proportional hazards regression to calculate Hazard Ratios (HRs) for invasive breast cancer risk after adjusting for previous trial participation (IBIS-II only), age, diabetes, hormone therapy use, smoking and menopausal status (IBIS-I only). Results: Higher BMI was associated with increased invasive breast cancer risk in IBIS-I (HR = 1.02, 95% CI = 1.00-1.03, p = 0.044) and IBIS-II (HR = 1.06, 95% CI = 1.03-1.09, p < 0.001). In IBIS-I, the association between BMI and breast cancer risk was restricted to postmenopausal women (HR = 1.03, 95% CI = 1.01-1.05, p = 0.004). Among IBIS-I postmenopausal women, compared with healthy weight, HRs were 1.20 (95% CI = 0.92-1.56) for overweight and 1.29 (95% CI = 0.97-1.72) for obese women. In IBIS-II HRs were 1.15 (95% CI = 0.75-1.75) for overweight and 1.90 (95% CI = 1.27-2.84) for obese women. There was no interaction between BMI and treatment group in either trial. Weight gain did not affect breast cancer risk. Conclusions: Contrary to NSABP P1 and P2 data, we found no evidence for increased breast cancer risk among overweight premenopausal women. IBIS-I and IBIS-II data suggest higher BMI increases breast cancer risk in postmenopausal women with elevated breast cancer risk. Clinical trial information: ISRCTN91879928 and ISRCTN31488319.