Key PointsQuestionDoes prophylactic lumbar cerebrospinal fluid drainage improve clinical outcomes measured by the modified Rankin Scale score among patients with aneurysmal subarachnoid hemorrhage?FindingsIn this pragmatic randomized clinical trial including 287 patients at 19 sites in 3 countries, the rate unfavorable neurologic outcome was 32.6% in the lumbar drainage group (47 of 144) and 44.8% in the standard of care group (64 of 143), a significant difference.MeaningIn this trial, among patients with aneurysmal subarachnoid hemorrhage, lumbar drainage improved clinical neurological outcomes at 6 months.