The effects of continuous drainage of cerebrospinal fluid (CSF) on vasospasm and hydrocephalus were analyzed retrospectively in 108 patients with subarachnoid hemorrhage (SAH) who were operated on for ruptured aneurysms within 48 hours of their onset. Ninety-two of these patients underwent a procedure for CSF drainage (cisternal drainage, ventricular drainage, lumbar drainage, or a combination of these). The duration, the total volume, and the average daily volume of CSF drainage were 10.4 +/- 7.0 days (mean +/- SD). 2034 +/- 1566 ml, and 190 +/- 65.3 ml, respectively. Patients with a greater drainage volume at a lower height of drainage in the early period after SAH developed more cerebral infarctions later (P less than 0.025). The relationship between the total volume of CSF removed and shunt-dependent hydrocephalus was determined to be statistically significant (P less than 0.005). Cerebral infarction and hydrocephalus after SAH were also found to be statistically associated (P less than 0.001). Thus, continuous cerebrospinal fluid drainage should not be performed too readily in patients with SAH, because the removal of a large amount of CSF can induce cerebral vasospasm as well as hydrocephalus.