OBJECTIVE: To investigate the effect of acute hypoxia and/or hypercapnia on cardio-ankle vascular index (CAVI) and blood pressure (BP) in patients with obstructive sleep apnea hypopnea syndrome (OSAHS). METHOD: CAVI and blood pressure were measured before and after isocapnic hypoxic, hyperoxia hypercapnic, and hypoxic and hypercapnic challenge in 28 non-hypertensive patients with OSAHS (AHI > 10/h) and 26 healthy controls (AHI < 5/h), respectively. They were matched for age and sex. Hypoxia and hypercapnia were induced by re-breathing technique. RESULTS: The 2 groups had no differences in regard to systolic (SBP) and diastolic BP(DBP) and CAVI. After hypercapnic challenge, SBP increased significantly in both groups. CAVI decreased significantly in controls, but not in OSAHS. Hypoxia induced significant increase of CAVI, but not in OSAHS. SBP and DBP maintained to the pre-challenge levels in both group. Hypercapnia and hypoxia together caused increase of SBP in both groups, and CAVI increased significantly in controls, but not in OSAHS. CONCLUSIONS: Acute hypoxia and hypercapnia exposure caused change of arterial stiffness and BP in both control and patients with sleep apnea hypopnea syndrome. However, CAVI responses to hypoxic and/or hypercapnic challenge were blunted in patients with OSAHS.