Intracranial pressure (ICP) was recorded continuously for 2 to days in 12 pre-operative patients with angiographic evidence of diffuse cerebral arterial spasm due to ruptured intracranial aneurysm. The ICP pattern of the low level (below 15 mmHg) and flat type was registered in the early stage of the arterial spasm in 11 patients in whom the ICP recording was made within 11 days after the hemorrhagic attack. During the period of low and flat ICP pattern, 6 patients showed little or no neurological deterioration whereas 5 patient showed impaired consciousness or neurological deficits. In 9 out 11 patients, the low level of ICP was followed by an abnormal deterioration. Continuous ventricular drainage was performed to control the secondary increased ICP in 7 patients and they survived, 4 of them with good results but remaining 3 with severe neurological deficits.