The incidence and clinical aspects of acute hydrocephalus were examined in 200 patients with recently ruptured intracranial aneurysms. The following conclusions were reached: Acute hydrocephalus is an important complication of aneurysmal subarachnoid hemorrhage that occurs in approximately 20% of all cases and exhibits an incidence that tends to parallel clinical grade (Grade I, 3%; Grade II, 5%; "Good" Grade III, 21%; "Bad"Grade III, 40%; Grade IV, 42%; Grade V, 26%). Impaired consciousness leading to a general downgrading of clinical status was the predominant clinical finding (93%), but neither this nor other nonspecific signs of increased intracranial pressure were distinguishable from the effects of the precipitating hemorrhage. The computed tomographic signs of acute hydrocephalus are distinctive and consist of selective ballooning of the frontal horns, rostral-caudal enlargement of the cerebral ventricles, and a halo of periventricular hyperdensity (edema) that evolves in sequence with ventricular changes. The treatment of choice is external ventricular drainage, which results in prompt and often dramatic improvement in approximately two-thirds of the patients.