Patients taking olanzapine, risperidone, or quetiapine with increased BMI of at least 10% over the previous 2 months of atypical therapy will be eligible for the study. Subjects will need to have been on a stable dose of their antipsychotic medications at the time of enrollment (stable x2 weeks) and will remain on the same atypical dose, if possible, throughout the study. The dosing of metformin will start at 500mg per day and increase up to a maximum dose of 1500mg daily. Determination of weight gain during the course of the study will be based on the subject's weight at enrollment and subsequent visits. The patients will be asked not to vary their dietary or physical activity habits during the study.