In obstructive sleep apnea syndrome (OSA) the periodic reduction or cessation of breathing due to narrowing or occlusion of the upper airway during sleep leads to an impaired cerebral vascular autoregulation that is associated with an increased cardiovascular risk, including stroke. Continuous positive airways pressure (CPAP) therapy at night is the most effective treatment for OSA and has been shown to reduce the cardiovascular risk in OSA patients. However, there is no suitable bedside monitoring method evaluating the recovery of cerebral hemodynamics during CPAP therapy. Near-infrared spectroscopy (NIRS) is ideally suited for non-invasive monitoring the cerebral hemodynamics during sleep due to its properties of local measurement, totally safe application and good tolerance to motion. In this pilot study, we monitored cerebral hemodynamics during standard CPAP therapy at night in three patients with severe OSA using NIRS. We found periodic oscillations in HbO2, HHb, tissue oxygenation index (TOI) and blood volume (BV) associated with periodic apnea events without CPAP in all OSA patients. These oscillations were eliminated under the optimal CPAP pressures in all patients. These results suggested that the recovery of cerebral hemodynamics impaired by apnea events can be evaluated by bedside NIRS measurements in real time during all night CPAP therapy. NIRS is a useful bedside monitoring tool to evaluate the treatment efficacy of CPAP therapy in patients with OSA.