Sixteen nondiabetic women with polycystic ovary syndrome (PCOS) aged 18 to 33 years were studied before and after 8 weeks on metformin (1.5 g/d) therapy to assess whether reducing hyperinsulinemia would reduce the levels of the major inhibitor of fibrinolysis, antigenic plasminogen activator inhibitor type 1 (PAI-1). Compared with six normal control women, PCOS women had a higher body mass index (BMI), waist to hip ratio, fasting insulin (Izero), insulin area under the curve during oral glucose tolerance testing (IA), glucose area under the curve during oral glucose tolerance testing (GA), IA/GA ratio, PAI-1, luteinizing hormone (LH) and ratio of LH to follicle-stimulating hormone (FSH), and free testosterone, and lower high-density lipoprotein (HDL) cholesterol (all P < .025). On metformin, BMI decreased 1.3% (P = .04), Izero 43% (P = .002), IA 31% (P = .03), GA 11% (P = .02), PAI-1 16% (P = .01), lipoprotein(a) [Lp(a)] 42% (P = .004), free testosterone 46% (P = .0006), LH 44% (P = .004), and the LH/FSH ratio 41% (P = .0001). On metformin, absolute and percent reductions in Izero correlated with absolute and percent reductions in PAI-1 (r = .60, P = .015 and r = .64, P = .008). On metformin, by stepwise multiple regression, the absolute reduction in Izero was a significant determinant of the absolute reduction in PAI-1 (partial R2 = 35%, P = .02), and the percent reduction in Izero was a significant determinant of the percent reduction in PAI-1 (partial R2 = 52%, P = .003). Metformin decreases Izero in hyperinsulinemic PCOS patients, reverses the hyperinsulinemia-driven endocrinopathy, decreases PAI-1, and decreases Lp(a), and should thus reduce the increased risk of atherothrombosis in PCOS.