OBJECTIVE: To report a case where insulin sensitization restored menses in an underweight woman with polycystic ovarian syndrome (PCOS). DESIGN: Case report. SETTING: Tertiary care center. PATIENT(S): A 19-year-old woman with a body mass index of 16.9 kg/m(2), severe hirsutism, and oligomenorrhea. INTERVENTION(S): Insulin sensitization with metformin. MAIN OUTCOME MEASURE(S): Impact of metformin therapy on menstrual cycle and serum T and fasting insulin levels. RESULT(S): Metformin, without weight loss or increased physical activity, resulted in restoration of menstrual cycle, reduction in serum T, and improvement in insulin resistance (IR). CONCLUSION(S): This case highlights the contribution of PCOS-related IR, distinct from visceral adiposity, and demonstrates the effectiveness of pharmacological insulin-sensitization independent of weight loss or lifestyle adjustments.