In postmenopausal women, BMI at age 18 years was not associated with risk of breast cancer overall although the OR was reduced more than 20% when comparing overweight and obese women to thin women (≥ 25 vs. < 20 kg/m2, OR = 0.79, 95% CI = 0.56–1.11; per 5 kg/m2 increase, OR = 0.98, 95% CI = 0.85–1.14); no differences in risk across ER/PR/HER2-defined subtypes was observed (P value for homogeneity of regression coefficients = 0.20). This evidence concerns the gene ERBB2 and breast cancer.