Spinal fusion surgery has increased dramatically and patients presenting for surgery are often more medically challenging. We hypothesized that advanced age and coexisting morbidities have increased in the population undergoing spinal fusion and are associated with greater risks for immediate complications and mortality. The Nationwide Inpatient Sample was retrospectively reviewed for discharges after a principal procedure code for elective spinal fusion. Total records meeting study inclusion criteria were 254,640. Coexisting morbidities were tabulated using Elixhauser comorbidities and the Charlson comorbidity index. Logistic regression identified risk factors associated with in-hospital mortality and early complications. The largest increase in spinal fusion surgery was in patients >65 years. Overall, those with at least 1 comorbidity increased (49% to 62%; P=0.002), as did mean Charlson index (0.146 to 0.202; P<0.001). In-hospital mortality was 0.13%, but 0.29%, and 0.64% for patients of 65 to 74, and those >or=75 years, respectively. Adjusted odds ratios for complications in 65-year to 74-year olds versus <65 years was 1.78 (95% confidence interval, 1.71-1.84; P<0.001), and for mortality 3.81 (95% confidence interval, 2.62-5.55; P<0.001); risks increased with the number of coexisting morbidities. Congestive heart failure, chronic pulmonary disease, coagulopathy, metastatic cancer, renal failure, and weight loss significantly correlated with in-hospital mortality, whereas hypertension or hypothyroidism had, unexpectedly, the opposite effect. Although it is known for some other forms of complex surgery, we showed that elderly and medically complex spinal fusion patients were at increased risk for in-hospital mortality and early complications. The majority of complications were operative, pulmonary, cardiovascular, or genito-urinary. Patient risk correlated with the number and nature of coexisting morbidities.