To learn whether metformin treatment affects oxidative stress as measured by serum concentrations of 8-hydroxy-2'-deoxyguanosine (8-OHdG).Double-blind, randomized, placebo-controlled trial.University outpatient clinic.The study cohort consisted of 50 obese women (body mass index [BMI] ≥ 27 kg/m(2)) and 60 nonobese patients (BMI <27 kg/m(2)), mean age was 27.7 ± 4.0 SD years.Randomization to receive metformin or placebo for 3 months.Serum levels of 8-OHdG before and after medical treatment.The levels of 8-OHdG were equal at baseline in the placebo and metformin groups. Obese women had higher baseline serum concentrations of 8-OHdG. Levels of 8-OHdG were statistically significantly reduced with metformin treatment, especially in obese patients with polycystic ovary syndrome. This study was a secondary subanalysis of a previously conducted prospective multicenter, randomized, placebo-controlled study on the effects of metformin on miscarriage, pregnancy, and miscarriage rates.Metformin treatment, compared with placebo, statistically significantly decreased 8-OHdG levels in women with polycystic ovary syndrome.NCT00994812.