We examined associations of prospective, validated information on body size in childhood and early adulthood, and midlife adiposity, with the risk of postmenopausal breast cancer and its main subtypes, as defined by histology, grade, and ER, progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) status, in a large study of UK women. This evidence concerns the gene ERBB2 and breast cancer.