Obesity is well established as a risk factor for postmenopausal breast cancer. Studies of body mass index (BMI) and breast cancer risk have found that postmenopausal women whose BMI falls in the obese category have about twice the breast cancer risk of women with a BMI in the normal weight category. However, obesity before menopause has been related to a weak decrease in premenopausal breast cancer risk while obesity after menopause has been related to a moderate increase in postmenopausal breast cancer. Other measures of body weight and body fat have been linked to an increase in the risk of postmenopausal breast cancer, including excess weight gain during adulthood, abdominal body fat distribution, and adult weight. Studies suggest that the underlying mechanism linking obesity to postmenopausal breast cancer is body fat. Obesity is one of few breast cancer risk factors that women can modify.