Overweight and obesity are well established risk factors for breast cancer that develop after menopause. The increased postmenopausal breast cancer risk in women who are overweight or obese is likely to be attributed to multiple metabolic disturbances. Metformin is a commonly used medication in diabetics to stabilize blood sugar. Association studies and laboratory studies have shown its potential to reduce the risk for development of cancer, including breast cancer. Recent pilot clinical studies in breast cancer patients suggest that metformin may only be effective in overweight or obese women with metabolic disturbances. We propose to conduct a clinical study of metformin in overweight or obese premenopausal women with metabolic disturbances. Study participants will be randomly assigned to receive metformin or placebo for 12 months. The study will evaluate whether metformin can result in favorable changes in risk features that have been associated with increased breast cancer risk. The risk features that will be examined in our study include breast density, certain proteins and hormones, products of body metabolism, and body weight and composition. The study should help determine the potential breast cancer preventive activity of metformin in a growing population at risk for multiple diseases.