BACKGROUND: Ischemic stroke occurring during sleep is still an unexplored area of cerebrovascular event. As the exact onset time of stroke while sleeping (SWS) cannot be determined, these patients are generally excluded from the thrombolytic therapy of acute ischemic stroke. The aim of this study was to know whether differences in clinical features exist between patients suffering a SWS and those with stroke while awake (SWA). METHODS: We reviewed the medical records of acute ischemic stroke patients consecutively registered in Hallym Stroke Databank between January 1999 and June 2007. We compared the risk factors and clinical features between the SWS and SWA groups. RESULTS: A total of 2,962 patients were included in the study, of which 821 (27.7%) were SWS. No differences between SWS and SWA were identified with regard to baseline clinical characteristics and risk factors except a history of smoking. In stroke subtype, small vessel occlusions were more frequently in SWS group than SWA group. Intravenous rt-PA treatments were performed frequently in the SWA group. Clinical outcomes at discharge were better in SWA group than SWS group. CONCLUSION: This study suggest that no major differences were exist in clinical characteristics between SWS and SWA patients, except the history of smoking. Clinical outcomes of patients with ischemic stroke within 6 hours after stroke onset were poor in SWS group. In SWS group, relatively little chances of thrombolysis might be the explanation of these finding.