Objective : Cerebral vasospasm still remains a major cause of the morbidity and mortality, despite the developments in treatment of aneurysmal subarachnoid hemorrhage. The authors measured the utility and benefits of external lumbar cerebrospinal fluid (CSF) drainage to prevent the clinical vasospasm and its sequelae after endovascular coiling on aneurysmal subarachnoid hemorrhage in this randomized study. Methods : Between January 2004 and March 2006, 280 patients with aneurysmal subarachnoid hemorrhage were treated at our institution. Among them, 107 patients met our study criteria. The treatment group consisted of 47 patients who underwent lumbar CSF drainage during vasospasm risk period (about for 14 days after SAH), whereas the control group consisted of 60 patients who received the management according to conventional protocol without lumbar CSF drainage. We created our new modified Fisher grade on the basis of initial brain computed tomography (CT) scan at admission. The authors established five outcome criteria as follows : 1) clinical vasospasm; 2) GOS score at 1-month to 6-month follow-up; 3) shunt procedures for hydrocephalus; 4) the duration of stay in the ICU and total hospital stay; 5) mortality rate. Results : The incidence of clinical vasospasm in the lumbar drain group showed 23.4% compared with 63.3% of individuals in the control group. Moreover, the risk of death in the lumbar drain group showed 2.1 % compared with 15% of individuals in the control group. Within individual modified Fisher grade, there were similar favorable results. Also, lumbar drain group had twice more patients than the control group in good GOS score of 5. However, there were no statistical significances in mean hospital stay and shunt procedures between the two groups. IVH was an important factor for delayed hydrocephalus regardless of lumbar drain. Conclusion : Lumbar CSF drainage remains to playa prominent role to prevent clinical vasospasm and its sequelae after endovascular coiling on aneurysmal subarachnoid hemorrhage. Also, this technique shows favorable effects on numerous neurological outcomes and prognosis. The results of this study warrant clinical trials after endovascular treatment in patients with aneurysmal SAH.