s u m m a r yBackground: The decision to perform surgery on older patients often presents as an ethical dilemma. The purpose of this study is to investigate the impact of comorbidities on the clinical outcomes of older rectal cancer surgery patients, with the goal of enabling healthcare professionals to evaluate the risk of surgery for the treatment of cancer in older patients with comorbid chronic disease. Methods: This study included 320 patients >60 years of age who were treated from 2004e2009 at a medical center in northern Taiwan. Logistic regression and Cox proportional hazards regression were used to determine if various chronic diseases (e.g., hypertension, diabetes, cardiovascular disease, cerebrovascular disease) increase the risk of in-hospital complications and 1-year mortality. Results: In the multivariate analysis, the risk of in-hospital complications for diabetic patients was 3.43 times that of nondiabetic patients (95% confidence interval [CI]: 1.13e10.37). For patients with cerebrovascular disease, the risk was 4.99 times that of those without cerebrovascular disease (95% CI: 1.30e19.07). In addition, patients 80 years of age demonstrated significantly higher 1-year mortality rates (HR Œ 3.49, 95% CI:1.18e10.30). However, a history of hypertension, diabetes, cardiovascular disease, or cerebrovascular disease was not a significant predictor of 1-year mortality. Conclusion: Older patients with comorbidities are at a higher risk of in-hospital complications following rectal cancer surgery, whereas the presence of comorbidities did not show a significant adverse effect on 1-year mortality in the present study. We suggest using population-based data to establish effective therapeutic strategies for treating each comorbidity.