Acute spontaneous subdural hematoma is very rare. We have encountered four such cases and verified the arterial origin of the bleeding at operation. None of the patients had a history of head trauma, and each had developed sudden onset of headache and other neurologic deficits, which simulate other cerebrovascular diseases. CT directly revealed subdural hematoma but gave no indication as to the source of the bleeding. Cerebral angiography was performed in all cases, with three of them showing localized extravasation of the contrast material into the subdural space. The extravasation was noted usually in the late arterial phase. This is a useful finding for diagnosing this disease and localizing the bleeding point. It is expected that with more routine use of cerebral angiography in cases of acute spontaneous subdural hematoma, extravasation of the contrast medium will be seen more frequently.