INTRODUCTION: Increasing costs and concerns about blood supply safety have led to a reevaluation of blood transfusion practices. This study was undertaken to examine blood use during aneurysm surgery. METHODS: We performed a retrospective analysis of hospital records including operative, anesthetic, and nursing notes, computed tomographic scans, and four-vessel angiographic films of 547 patients undergoing surgery for ruptured and unruptured cerebral aneurysms at Harborview Medical Center in Seattle. During the review period, the transfusion threshold was not altered. RESULTS: A total of 134 patients (24.5%) received an intraoperative blood transfusion (median number of units, 2; range, 1-17). Preoperative factors associated with intraoperative blood use included older patient age (P < 0.001), lower hematocrit level on admission (P = 0.007), ruptured rather than unruptured aneurysm (P = 0.004), severe intraventricular hemorrhage (P = 0.03), and larger aneurysm size (P = 0.004). Factors not associated with intraoperative blood transfusion included past medical history (including cardiac or pulmonary disease), admission clinical grade after aneurysm rupture, findings such as hydrocephalus on computed tomographic scanning, and aneurysm location and aneurysm neck-to-fundus ratio. Also associated with blood transfusion during surgery were intraoperative aneurysm rupture (P < 0.0001), intracerebral hematoma evacuation (P = 0.02), and obliteration of multiple aneurysms (P = 0.002). Among patients who received an intraoperative transfusion, those who experienced an aneurysm rupture required an average of 3.6 +/- 0.35 units, whereas patients who did not have a rupture required 1.9 +/- 0.12 units (P = 0.001). Postoperatively, a total of 244 patients (44.6%), including 77 who received blood intraoperatively, required a blood transfusion (median number of units, 2; range, 1-31). Postoperative blood transfusion was associated with the treatment of patients with subarachnoid hemorrhage (P < 0.0001), particularly among poor-grade patients who developed medical complications. CONCLUSION: Blood transfusion can be expected in one in five patients undergoing aneurysm surgery. Reducing intraoperative rupture may reduce the need for blood products.