Given the widespread use of continuous external ventricular drainage in patients with aneurysmal subarachnoid hemorrhage (SAH), this investigation was undertaken to define the relationship of ventricular drainage to aneurysmal rebleeding. A historical cohort study of 128 patients with confirmed aneurysmal SAH was performed using a multivariate stepwise logistic regression analysis to examine the relationship between aneurysmal rerupture and ventricular drainage, while controlling for important clinical and radiological independent variables. The variables for ventricular drainage selected in the regression analysis were clinical grade, aneurysm size, and presence of hydrocephalus. The rate of rerupture was significantly higher in cases with ventricular drainage (odds ratio 5.31:1, p less than 0.05), poor clinical grade (odds ratio 4.90:1, p less than 0.02), and large aneurysm size (odds ratio 11.25:1, p less than 0.01). The significant effect of ventricular drainage was limited to patients with hydrocephalus. The increased risk of aneurysmal rebleeding in patients undergoing ventricular drainage may result from both: 1) a rise in aneurysmal transmural pressure, since intracranial pressure is lowered by ventricular drainage; and 2) an association between ventricular drainage and a more severely disrupted aneurysm which is more prone to rebleed as part of its natural history. This study found an increased risk of aneurysmal rebleeding among patients undergoing ventricular drainage, particularly in the presence of hydrocephalus.