BACKGROUND AND PURPOSE: Obstructive sleep apnea syndrome (OSAS) is an independent risk factor for stroke. Impairment of cerebral autoregulation may play a potential role in the pre-disposition to stroke of OSAS patients. In this study, we aimed to assess dynamic cerebral autoregulation (DCA) during wakefulness in OSAS patients and a group of matched controls. METHODS: Patients and controls were examined in the morning after an overnight complete polysomnography. Mean cerebral blood flow velocity (CBFV) in the middle cerebral artery and mean arterial blood pressure (ABP) were continuously recorded using transcranial Doppler and Finapres. DCA was assessed using the Mx autoregulatory index. Mx is a moving correlation coefficient between mean CBFV and mean ABP. More positive value of Mx indicates worse autoregulation. RESULTS: Eleven OSAS patients (mean age +/- SD; 52.6 +/- 7.9) and 9 controls (mean age +/- SD; 49.1 +/- 5.3) were enrolled. The mean apnea-hypopnea index (AHI) in the OSAS group was of 22.7 +/- 11.6. No significant difference was found between the two groups as for age, body mass index, mean ABP and endtidal CO(2) pressure. Cerebral autoregulation was impaired in OSAS patients compared with controls (Mx index: 0.414 +/- 0.138 vs. 0.233 +/- 0.100; P = 0.009). The severity of autoregulation impairment correlated to the severity of the sleep respiratory disturbance measured by the AHI (P = 0.003). CONCLUSION: Cerebral autoregulation is impaired in patients with OSAS during wakefulness. Impairment of cerebral autoregulation is correlated with the severity of OSAS.