Previous research has shown that persons undergoing certain high-risk surgical procedures at high-volume hospitals (HVHs) have a lower risk of postoperative death than those undergoing surgery at low-volume hospitals (LVHs). This study estimated the absolute number of operative deaths that could potentially be avoided if 5 major surgical procedures in Ontario were restricted to HVHs. Data was collected on all persons who underwent esophagectomy (613), colon or rectal resection for colorectal cancer (18 898), pancreaticoduodenectomy (686), pulmonary lobectomy or pneumonectomy for lung cancer (5156) or repair of an unruptured abdominal aortic aneurysm (AAA) (6279) in Ontario from Apr. 1, 1994, to Mar. 31, 1999. The excess number of operative deaths (defined as deaths in the period from the day of the operation to 30 days thereafter), adjusted for age, sex and comorbidity, among the 75% of persons treated in LVHs, as compared with the 25% treated in the highest-volume quartile of hospitals was calculated. Bootstrap methods were used to estimate 95% confidence intervals (CIs). Of the 31 632 persons undergoing any of the 5 procedures, 1341 (4.24%) died within 30 days of surgery. If the 75% of persons treated at the LVHs had instead been treated at the HVHs, the annual number of lives potentially saved would have been 4 (95% CI, 0 to 9) for esophagectomy, 6 (95% CI, 1 to 11) for pancreaticoduodenectomy, 1 (95% CI, -10 to 13) for major lung resection and 14 (95% CI, 1 to 25) for repair of unruptured AAA. For resection of colon or rectum, the regionalization strategy would not have saved any lives, and 17 lives (95% CI, 36 to -3) would potentially have been lost. A small number of operative deaths are potentially avoidable by performing 4 of 5 complex surgical procedures only at HVHs in Ontario. In determining health policy, the most compelling argument for regionalizing complex surgical procedures at HVHs may not be the prevention of a large number of such deaths.