BACKGROUND: Few data are available that systematically describe rates and trends of postoperative mortality for fairly large, unselected patient populations. METHODS: This population-based study uses a registry of 3.7 million surgical procedures in 102 hospitals in The Netherlands during 1991-2005. Patients older than 20 yr who underwent an elective, nonday case, open surgical procedure were enrolled. Patient data included main (discharge) diagnosis, secondary diagnoses, dates of admission and discharge, death during admission, operations, age, sex, and a limited number of comorbidities classified according to the International Classification of Diseases 9th revision Clinical Modification. The main outcome measure was postoperative all-cause mortality. Univariable and multivariable logistic regression analyses were applied to evaluate the relationship between type of surgery and the main outcome. RESULTS: Postoperative all-cause death was observed in 67,879 patients (1.85%). In a model based on a classification into 11 main surgical categories, breast surgery was associated with lowest mortality (adjusted incidence, 0.07%), and vascular surgery was associated with highest mortality (adjusted incidence, 5.97%). In a model based on 36 surgical subcategories, the adjusted mortality ranged from 0.07% for hernia nuclei pulposus surgery to 18.5% for liver transplant. The c-index of the 36-category model was 0.88, which was significantly (P < 0.001) higher than the c-index that was associated with the simple surgical classification (low vs. high risk) in the commonly used Revis