Objective: To test the hypothesis that adrenal androgen (AA) excess in the polycystic ovary syndrome (PCOS) is due to a generalized exaggeration in AA output in response to adrenocorticotropic hormone (ACTH), and that this abnormality is due to an identifiable alteration in the biosynthesis of AAs. Design: Cross-sectional prospective controlled study. Setting: Academic tertiary care medical center. Patient(s): Patients with PCOS (n = 9) and without (n = 9) AA excess and controls (n = 12) without hyperandrogenism, matched for age and body mass. Intervention(s): Acute 60-minute ACTH test was performed on patients. Main Outcome Measure(s): Basal levels of dehydroepiandrosterone sulfate (DHEAS), total testosterone (T), free T, and basal (Steroid0) and the 60-minute ACTH-stimulated levels (Steroid60) of pregnenolone (PREG), progesterone (P4), 17-hydroxypregnenolone (17-HPREG), 17-hydroxyprogesterone (17-HP), dehydroepiandrosterone (DHEA), and androstenedione (A4) were measured. Adrenocortical activities of 17-hydroxylase (17-OH), 17,20-lyase, and 3β-hydroxysteroid dehydrogenase were estimated from product to precursor ratio, using Steroid60 values. Result(s): Compared with PCOS patients without AA excess, PCOS patients with AA excess demonstrated significantly greater levels of DHEA0 and A460. PCOS patients with AA excess had significantly higher activity of Δ 517-OH, compared with PCOS patients without AA excess. Conclusion(s): Adrenal androgen excess in PCOS is associated with a greater Δ 517-OH activity in response to ACTH. © 2004 by American Society for Reproductive Medicine.