In adults, metformin promotes weight loss and prevents the development of type 2 diabetes mellitus (DM2). However, these effects have not been demonstrated in adolescents at risk for DM2. Objective: To analyze the anthropometric and metabolic impact of metformin in obese adolescents at risk for DM2. Patients and Methods: A double-blind, placebo-controlled study was conducted in 19 obese female adolescents at risk for DM2. A structured lifestyle intervention with nutritional and exercise education and motivational support was assessed over 3 month with an additional follow up period of 3 months. Subjects were randomized to 500 mg/ daily of extended release metformin or placebo. Anthropometric (weight, BMI, waist circumference, blood pressure) and metabolic profiles (glycemia, HOMA, lipid profile, AST, ALT) were compared between both groups at the end of both periods. Results: Metformin treated group showed a significant reduction in weight and body mass index (BMI) compared with placebo group. No improvement in the metabolic risk profile was showed in any group. Conclusion: In this study, metformin therapy in combination with a lifestyle intervention helps to reduce weight and BMI in obese adolescent females at risk for DM2, compared to lifestyle and placebo intervention.