The identification of potentially modifiable risk factors, such as body size, could allow for interventions that could help reduce the burden of contralateral breast cancer (CBC) among breast cancer survivors. Studies examining the relationship between body mass index (BMI) and CBC have yielded mixed results. From the population-based, case-control, Women's Environmental, Cancer and Radiation Epidemiology (WECARE) Study, we included 511 women with CBC (cases) and 999 women with unilateral breast cancer (controls) who had never used postmenopausal hormone therapy. Rate ratios (RR) and 95% confidence intervals (CI) were used to assess the relationship between BMI and CBC risk. No associations between BMI at first diagnosis or weight-change between first diagnosis and date of CBC diagnosis (or corresponding date in matched controls) and CBC risk were seen. However, obese (BMI ≥ 30 kg/m(2)) postmenopausal women with estrogen receptor (ER)-negative first primary tumors (n = 12 cases and 9 controls) were at an increased risk of CBC compared with normal weight women (BMI < 25 kg/m(2)) (n = 43 cases and 98 controls) (RR = 5.64 (95% CI 1.76, 18.1)). No association between BMI and CBC risk was seen in premenopausal or postmenopausal women with ER-positive first primaries. Overall, BMI is not associated with CBC risk in this population of young breast cancer survivors. Our finding of an over five-fold higher risk of CBC in a small subgroup of obese postmenopausal women with an ER-negative first primary breast cancer is based on limited numbers and requires confirmation in a larger study.