Objective: To determine whether the severity in hyperandrogenemia determines, to a significant degree, the severity of hirsutism in patients with polycystic ovary syndrome (PCOS). Design: Cross-sectional study. Setting: Tertiary care academic referral center. Patient(s): A total of 749 patients with PCOS. Intervention(s): History and physical examination, blood sampling. Main Outcome Measure(s): Hirsutism defined by a value of ≥6 using the modified Ferriman-Gallwey (mFG) score, age, body mass index (BMI), calculated homeostatic assessment for insulin resistance (HOMA-IR) value, and levels of total (TT) and free (FT) testosterone, dehydroepiandrosterone sulfate (DHEAS) 17-hydroxyprogesterone (17OH-P), and fasting insulin (INS) and glucose (GLC). Result(s): Univariate correlations revealed associations between the mFG score and INS, 17OH-P, HOMA-IR, and BMI. Multivariate classification and regression tree analysis indicated that INS had the most significant association with mFG score and that at higher INS levels T played an additional role whereas at lower INS levels 17OH-P had an effect; however, this model accounted for only 8.2% of total variation in mFG score. Conclusion(s): Insulin appears to have a direct effect on the severity of hirsutism in PCOS and appears to have a synergistic interaction with TT. Notably, over 90% of the variation in the mFG score was not related to the factors studied and likely reflects intrinsic factors related to pilosebaceous unit function or sensitivity and to other factors not yet assessed. © 2009 American Society for Reproductive Medicine.