We reviewed the results of ventriculostomy with external ventricular drainage in patients with acute hydrocephalus complicating subarachnoid haemorrhage. Of 194 consecutive patients with subarachnoid haemorrhage admitted during the past eight years, 52 (27%) developed hydrocephalus within 72 hours of the ictus. Patients with acute hydrocephalus were in grades III to V (Hunt and Hess) at the time of evaluation and all patients with hydrocephalus underwent ventriculostomy within 24 hours of diagnosis. Twenty-six patients improved within 24 hours of cerebrospinal fluid drainage and 17 of these patients underwent surgery, nine of whom did well (Glasgow Outcome Scale 1 and 2). All 18 patients who did not improve within this period, including one who worsened, died. In eight patients the response to ventriculostomy was considered as undetermined, because of the proximity of the drain insertion to a definitive surgical procedure, and all of them had an excellent outcome (Glasgow Outcome Scale 1). Of 32 patients in grades IV and V, 17 did not improve and all of them died. Eight of the 15 patients in these grades, who were in the improved or undetermined categories, did well. Five patients (10%) developed meningitis. All patients with this complication had drainage for more than four days. Seven patients (14%) had a rebleed during the drainage. All except one patient with a rebleed had no surgery or delayed surgery and in six of them recurrent haemorrhages occurred after more than 24 hours of drainage. We conclude that routine ventriculostomy with external ventricular drainage should be considered for all patients with altered sensorium and acute hydrocephalus following subarachnoid haemorrhage.(ABSTRACT TRUNCATED AT 250 WORDS)