The objective of the present study was to investigate whether metformin affected plasma concentrations of insulin-like growth factor (IGF) I and IGF-binding protein I (IGFBP-I) in polycystic ovary syndrome (PCOS) patients. This was an open study conducted by the Department of Obstetrics and Gynecology at the University of Siena, Italy. Seventeen women with PCOS participated in the study and were administered metformin at a dose of 500 mg three times a day. Treatment was continued for 30-32 days, after which the pretreatment evaluation was repeated. Plasma concentrations of LH, FSH, estradiol, free testosterone, IGF-I, IGFBP-I, sex hormone-binding globulin, and insulin were evaluated. Metformin led to a significant reduction in areas under the insulin curves (9310 ± 1509 vs. 6520 ± 1108 mU/mL·min; P < 0.05) and was associated with a decrease in plasma free testosterone levels (12.7 ± 1.7 vs. 10.3 ± 2 pg/mL; P < 0.05) and an increase in plasma sex hormone-biding globulin concentrations (62 ± 8 vs. 94 ± 13 nmol/L; P < 0.05). A nonsignificant increase in plasma IGF-I levels was observed after metformin (276 ± 48 vs. 291 ± 71 mcg/L), with a significant increase in plasma IGFBP-I levels (0.56 ± 0.2 vs. 0.98 ± 0.38 mcg/L; P < 0.05). The IGF-I/IGFBP-I ratio was significantly lower (492.8 ± 117 vs. 296.9 ± 82; P < 0.05) at the end of therapy than before treatment. In conclusion, it seems to be appropriate to intervene with an insulin-sensitizing agent such as metformin in an attempt to break the pathogenetic link between hyperinsulinemia and hormonal perturbations in PCOS.