Intraventricular pressure (IVP) was measured continuously by the method of Lundberg for an average period of 8 days in 52 patients with recent rupture of an intracranial saccular aneurysm. The patients were graded as follows according to the system of Hunt and Hess: 13 patients were Grades I-II, 19 patients Grades II-III, and 20 patients Grades III-V. The degree of cerebral arterial spasm was measured on angiograms taken on admission and approximately 7 days later. Drainage of ventricular cerebrospinal fluid was performed intermittently when IVP exceeded 25 mm Hg. The results showed a close correlation between changes in clinical grade and in mean IVP. Drainage improved the condition in uncomplicated cases, but was less effective or ineffective when severe vasospasm or rebleeding occurred. Lowering the IVP by drainage did not appear to increase the risk of rebleeding. In contrast to patients with no or slight spasm, patients with severe spasm had a permanently elevated IVP, even before spasm was demonstrable angiographically. A mean IVP exceeding 25 mm Hg for the whole period of monitoring was associated with a poor prognosis. Finally, indications for continuous monitoring of IVP and ventricular drainage in aneurysm patients are suggested.