OBJECTIVE: To evaluate the effect of metformin addition during ovarian hyperstimulation on cycle parameters and outcome of in vitro fertilization (IVF) treatment in polycystic ovary syndrome (PCOS) patients with a body mass index (BMI) <28 kg/m(2). DESIGN: Retrospective review of patients' records. SETTING: Istanbul Memorial Hospital Assisted Reproductive Treatment Unit. POPULATION: A total of 339 non-obese PCOS patients undergoing IVF were evaluated according to the supplementation of metformin. METHODS: Cycle parameters and IVF outcomes of 220 patients given metformin were compared to those of 119 patients treated without metformin. MAIN OUTCOME MEASURES: Implantation and pregnancy rates. RESULTS: Metformin co-treatment led to significantly lower peak E2 levels (3,481 pg/ml vs. 4,192 pg/ml; p <0.0001). However, gonadotropin consumption, stimulation duration, numbers of total and mature oocytes retrieved, fertilization rate, and ratio of grade I embryos developed were similar in both groups. In the metformin administered group, significantly higher implantation (25% vs. 18%; p=0.003) and pregnancy rates (58% vs. 45%; p=0.04) were achieved. Abortion and moderate-severe ovarian hyperstimulation syndrome rates were found to be similar in both groups. CONCLUSIONS: Although metformin use was especially and strongly recommended in obese PCOS cases, its supplementation during IVF in PCOS patients with a BMI <28 kg/m(2) was observed to be beneficial and efficacious.