Purpose : To evaluate the characteristic CT and cerebral angiographic findings in patients with acutespontaneous subdural hematomas and correlate these imaging findings with causes of bleeding and clinical outcome.Materials and Methods : Twenty-one patients with nontraumatic acute spontaneous subdural hematoma presentingduring the last five years underwent CT scanning and cerebral angiography was performed in twelve. To determinethe cause of bleeding, CT and angiographic findings were retrospectively analysed. Clinical history, laboratoryand operative findings, and final clinical outcome were reviewed. Results : The 21 cases of acute spontaneoussubdural hematomas were caused by cerebral vascular abnormalities(n=10), infantile hemorrhagic disease(n=5), orwere of unknown origin(n=6). All ten cases of cerebral vascular abnormality were confirmed angiographically; sixwere aneurysms, three were arteriovenous malformations, and one was moyamoya disease. On CT, subarachnoidhemorrhage was seen to be associated with aneurysms, intracerebral hemorrhage with arteriovenous malformations,and intraventricular hemorrhage with moyamoya disease. All five patients with hemorrhagic disease were infantsaged 1-17 months ; characteristic diffuse distribution of subdural hematoma in both temporoparietal-occipitalregions is typical. The average overall mortality rate was 52.4%(11/21). In patients with cerebral vascularabnormalities, mortality was as low as 20%(2/10), but in hemorrhagic disease was high (60%). In cases of unknownorigin it was 100%. Conclusion : Acute spontaneous subdural hematoma is a rare condition, and the mortality rateis high. In patients with acute spontaneous subdural hematoma, as seen on CT, associated subarachnoid orintracerebral hemorrhage is strongly indicative of intracerebral vascular abnormalities such as aneurysm andarteriovenous malformation, and cerebral angiography is necessary. To ensure proper treatment and thus morkedlyreduce mortality, the causes of bleeding should be prompty determined by means of cerebral angiography.