Pre-existing obesity and postoperative weight gain are related to a poor prognosis in breast cancer regardless of menopausal status. Delayed diagnosis may be one cause, but of more biological significance, especially in younger women, is the association of adiposity with estrogen receptor-negative tumors with a propensity for distant metastasis. After the menopause, the major mechanism for the relationship is the elevated estrogen synthesis by adipose tissue; these hormone-dependent tumors are estrogen receptor-positive. Insulin and some adipokines also stimulate breast cancer growth and metastasis, both directly and most probably by enhanced angiogenesis. Weight control is important, not only to target breast cancer progression, but also to reduce the risk of nonbreast cancer mortality risk associated with excess adiposity.