Background and PurposeAfter a craniotomy for cerebral aneurysm, postoperative mortality can be significant. Previous studies have shown that hospitals performing frequent highrisk procedures (such as coronary artery bypass) have a lower mortality than hospitals where these procedures are performed infrequently.MethodsThe Statewide Planning and Research Cooperative System of the New York State Department of Health reviewed all discharges in New York State from 1987 through 1993 for the diagnoses of subarachnoid hemorrhage and/or cerebral aneurysm and for patients with the procedure code for craniotomy for ruptured or unruptured cerebral aneurysm. Inhospital mortality and length of stay were examined in relation to the volume of craniotomies for aneurysm performed at each individual hospital.Results30 craniotomies per year for cerebral aneurysm compared with hospitals performing less surgery (8.8% versus 15.5%, P < .0001). For all patients who underwent craniotomy for unruptured cerebral aneurysm (n = 1604), there was an identical 43% (95% confidence interval, 14% to 73%) reduction in mortality in hospitals performing more than 30 craniotomies per year for cerebral aneurysm (4.6% versus 8.1%, P = .0087).ConclusionsHospitals that frequently perform aneurysm operations have lower mortality rates for patients undergoing craniotomy for cerebral aneurysm than hospitals that perform fewer operations.(Stroke. 1996;27:18-23.)