Objective: The purpose of this study was to assess the sibling correlation of adrenocorticotropic hormone-stimulated steroid hormone levels between probands with polycystic ovary syndrome and their sisters. Study Design: Twenty-seven women with polycystic ovary syndrome and 28 of their sisters underwent an adrenocorticotropic hormone stimulation test with measurement of the steroids dehydroepiandrosterone, androstenedione, and cortisol 60 minutes later. The 60-minute values were used to calculate sister-sister correlations by regression analyses. Results: The adrenocorticotropic hormone-stimulated (60-minute) log-transformed dehydroepiandrosterone and cortisol values were significantly correlated between siblings (r = 0.47, P = .01 and r = 0.57, P = .01, respectively); a similar trend was observed for the 60-minute A4 values (r = 0.29, P = .06). Conclusion: Women with polycystic ovary syndrome and their sisters have significantly correlated levels of adrenocorticotropic hormone-stimulated steroids, supporting a genetic basis of the adrenal androgen excess observed in polycystic ovary syndrome. © 2007 Mosby, Inc. All rights reserved.