PCOS subjects will be screened by questionnaire for eligibility with the goal of recruiting 40 subjects who are 14 to 25 years, BMI between the 10th and 95th percentile for age (<20 years) and 18.5 - 29.9 kg/m2 (20 years or older), and at least two years post-menarche. Oligomenorrheic subjects will complete the protocol during the early follicular phase of the menstrual cycle (days one through seven) and amenorrheic subjects will complete the protocol on any day. The protocol will comprise: a) History and physical exam: Height, weight, body mass index (BMI) (percentile and z-score for <20 years), blood pressure, physical exam, Ferriman-Gallwey hirsutism scoring, smoking, alcohol and family history; b) Laboratory evaluation will be between 0800 and 0900 after an overnight fast via a 21 gauge intravenous catheter for: 1) General endocrine panel: Thyroid function, prolactin, 17-hydroxyprogesterone; 2) PCOS panel: luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol, sex hormone binding globulin, testosterone, free testosterone, androstenedione, dehydroepiandrosterone; 3) Liver panel: Liver enzymes (alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma glutamyl-transpeptidase (GGT)), apoptosis markers (Fas, cytokeratin-18 fragments, M30), pancreatic polypeptide; 4) Cardiovascular risk panel: Cholesterol, triglycerides, HDL, LDL, free fatty acids, c-reactive protein; 5) IR evaluation: hemoglobin A1C, glucose, insulin, c-peptide and two-hour oral glucose tolerance test (OGTT) after a 75 gram glucose challenge with measurement of glucose and insulin at 30, 60, 90 and 120 minutes. IR will be determined by age- and BMI- specific homeostatic measure of insulin resistance (HOMA-IR) cutoffs based on a National Health and Nutrition Examination Survey (NHANES) study of 1,164 nonobese adolescents 12 to 19 years old. Other IR indices that will be evaluated are whole body insulin sensitivity (WBIS) and insulin area under the curve; 6) Genetic evaluation: A blood sample for the PNPLA3 I148M allele (baseline only). c) Magnetic resonance spectroscopy (MRS) of the liver for intrahepatic lipid content acquired using standard point resolved spectroscopy sequence (PRESS). Percentage liver fat will be calculated. d) Total body magnetic resonance imaging (MRI) with 10 mm slices and 40 mm interslice gaps for total body adipose tissue and VAT. e) Dual-energy x-ray absorptiometry (DXA): Total body DXA will be performed for total body lean and fat mass and percentage body fat. f) Randomization: Subjects whose liver fat is 4.8% or greater (n≈20) will be randomized to placebo or extended-release metformin 1000 mg (2 tablets) daily. Randomization will be double-blind and will be performed by randomly selected permuted size 2 and size 4 blocks determined by the Columbia University Medical Center (CUMC) Research Pharmacy with the guidance of a statistician. g) Intervention: All subjects will receive identically appearing pill bottles labeled "Study Medication" containing their first three-month supply of either metformin or placebo. Subjects will be instructed to take pills with dinner and to take one pill once a day for one week and subsequently two pills once a day for the remainder of the first three months. Subjects will be asked to call with any side effects that they may experience. They will receive weekly calls from a research coordinator and will be asked to bring their empty pill bottles to their follow-up studies do to ensure compliance. h) Brief interim evaluation: After three months of the intervention phase, subjects will be seen for a general health assessment, which includes weight, blood pressure and a physical examination, and will receive their second three-month supply of their assigned intervention. i) Follow-up exam: At the end of six months, subjects will undergo laboratory and body composition testing identical to those performed in the baseline testing.