The positive association between obesity and postmenopausal breast cancer has been attributed, in part, to the fact that estrogen, a risk factor for breast cancer, is produced in fat tissue. Obesity is also associated with high levels of insulin, a factor known to stimulate the replication of cells. This report from Howard Strickler’s research group at AECC is the first prospective study to show, directly, that insulin levels are strongly associated with the risk of breast cancer independent of the estrogen level. Since high insulin levels are associated with obesity and type II diabetes, these observations help elucidate the basis for the increased incidence of cancer in obese women. The study also identifies the insulin signaling pathway as a potential target for the development of new approaches to the chemoprevention of breast cancer.