Overweight children 6 to 11 years old who are in general good health may be eligible for this study. Children will be studied at the National Institutes of Health in Bethesda, Maryland. Candidates will have a medical history and physical examination and fasting blood test, and will provide a 7-day record of their food intake as part of the screening process. Those enrolled will be randomly assigned to receive either metformin or placebo (a look-alike tablet with no active medicine) twice a day for a six month period. After the 6 month study period, all children will be offered the opportunity to take metformin for another 6 months.