OBJECTIVE: To investigate comorbid conditions with prognostic influence in non - ST-segment elevation acute coronary syndrome (NSTEACS). PATIENTS AND METHODS: The study group consisted of a derivation cohort of 1017 patients (admitted from October 1, 2002, through October 1, 2008) and an external validation cohort of 652 patients (admitted from February 1, 2006, through September 30, 2009). Comorbid conditions, including risk factors and components of the Charlson comorbidity index (ChCI) and coronary artery disease - specific index, were recorded. The main outcome was one-year mortality. RESULTS: During follow-up, 103 patients died. After adjusting for variables associated with NSTEACS characteristics (base model), 5 comorbid conditions predicted mortality: severe or mild renal failure (hazard ratio [HR], 2.9 and HR, 1.6, respectively), dementia (HR, 3.1), peripheral artery disease (HR, 2.0), previous heart failure (HR, 2.6), and previous myocardial infarction (HR, 1.4). A simple comorbidity index (SCI) was developed using these variables, (per point: HR, 1.6; 95% confidence interval, 1.4-1.8; P=.0001). Adding the SCI, Charlson comorbidity index, or coronary artery disease - specific index to the base model resulted in a gain of 6.58%, 5.00%, and 4.04%, respectively, in discriminative ability (P=.001), without significant differences among the 3 indices. In patients with comorbid conditions, the highest risk period was in the first weeks after NSTEACS. The strength of the association between SCI and mortality rate was similar in the external validation cohort (HR, 1.3; 95% confidence interval, 1.1-1.6; P=.001). CONCLUSION: Renal dysfunction, dementia, peripheral artery disease, previous heart failure, and previous myocardial infarction are the comorbid conditions that predict mortality in NSTEACS. A simple index using these variables proved to be as accurate as the more complex comorbidity indices for risk stratification. Inhospital management of patients with comorbid conditions merits further investigation. © 2011 Mayo Foundation for Medical Education and Research.