Although polycystic ovary syndrome is associated with hypertension, hyperlipidaemia, and insulin resistance, mortality from cerebrovascular disease is not increased. We previously reported lower downstream resistance in the internal carotid artery in women with polycystic ovary syndrome. This study was designed to assess vascular reactivity by measuring the response to inhalation of 5% carbon dioxide. We studied 34 young women with polycystic ovary syndrome, 15 with symptomless polycystic ovaries, and 18 controls.