OBJECTIVE: To evaluate insulin sensitivity (IS) and beta-cell function (beta F) in lean and obese women with polycystic ovary syndrome (PCOS), either separately or by using a disposition index (DI). RESEARCH DESIGN AND METHODS: A total of 64 women with PCOS and 20 healthy women were examined by anthropometry, oral glucose tolerance tests (OGTTs), and insulin tolerance tests. Statistical analysis used one-way ANOVA, Kruskal-Wallis, and Mann-Whitney U tests, as appropriate. RESULTS: A significantly higher waist-to-hip ratio (P < 0.0001) was found in both lean and obese women with PCOS. Higher basal blood glucose (P < 0.004) and blood glucose values at 3 h of OGTT (P < 0.008) were found in lean and obese PCOS subjects in comparison with control subjects. Insulin resistance by homeostasis model assessment (P < 0.007) was significantly higher in obese PCOS than in control or lean PCOS subjects. Early-phase insulin secretion (insulinogenic index [Delta I/Delta G(30-0), where I is insulin and G is glucose]; P < 0.0007) was significantly higher in both lean and obese PCOS subjects than in healthy women. All tested combinations of parameters of IS and beta F (DIs) followed a physiological hyperbolic relationship. Significantly lower values of the fasting state-derived DIs were found (all P < 0.05) in obese PCOS subjects. Significantly higher values of all of these indexes derived from nonfasting values were found in lean PCOS as compared with control and obese PCOS subjects (all P < 10(-3)). CONCLUSIONS: Increased beta F was found even in lean individuals with PCOS. Insulin hypersecretion is thus probably connected to the pathogenesis of PCOS.