Computed tomography demonstrated acute hydrocephalus less than or equal to 72 hours after subarachnoid hemorrhage in 24 (23%) of 104 patients. Of these 24 patients, six (25%) had no impairment of consciousness. In nine (11%) of the remaining 80 patients, acute hydrocephalus developed within 1 week after subarachnoid hemorrhage. With the exception of three patients, all 104 patients received antifibrinolytic treatment. Delayed clinical deterioration from acute hydrocephalus occurred in seven (29%) of the 24 patients with acute hydrocephalus on admission and in six (8%) of the remaining 80 patients. Serial lumbar puncture was performed in 17 patients. Twelve (71%) of the 17 patients treated with serial lumbar puncture, including 10 (77%) of the 13 patients with delayed deterioration from acute hydrocephalus after admission, achieved improvement in the level of consciousness. Four of these 17 patients (4% of all 104 patients) required an internal shunt. No patient deteriorated from coning following serial lumbar puncture. The rebleeding rate within 12 days after subarachnoid hemorrhage in hydrocephalic patients with serial lumbar puncture was not higher than the rate in those without hydrocephalus (two [12%] of 17 versus nine [13%] of 71). Neither meningitis nor ventriculitis was observed. We conclude that if neither a hematoma with a mass effect nor an obstructive element exists, cerebrospinal fluid drainage with serial lumbar puncture is a good alternative to ventricular drainage in patients with acute hydrocephalus after subarachnoid hemorrhage.