BACKGROUND: Older patients often suffer from multiple comorbid conditions. Few comorbidity indices are valid and reliable in the elderly and were rarely compared. OBJECTIVE: To compare the performance, relevance, and ability of 6 widely used and validated comorbidity indices-Charlson Comorbidity Index, Cumulative Illness Rating Scale-Geriatrics, Index of Coexistent Diseases, Kaplan, Geriatric Index of Comorbidity (GIC), and Chronic Disease Score-to predict adverse outcomes after discharge (1-year risk of rehospitalization, institutionalization, and death). DESIGN, SETTING, AND PARTICIPANTS: Prospective study with 1-year follow-up, between January 2004 and December 2005 in 444 elderly patients (mean age, 85; 74% female) discharged from acute geriatric hospital, Geneva University Hospitals. RESULTS: In univariate analyses, Cumulative Illness Rating Scale?Geriatrics and GIC were the predictors with the largest coefficient of determination for mortality with (R(2) of 9.3%, respectively 8.8%). GIC was also the only significant predictor of institutionalization (R(2) = 6.0%). Higher risk of readmission was significantly associated with GIC (R(2) = 14.0%), Cumulative Illness Rating Scale-Geriatrics (R(2) = 5.6%), Charlson Comorbidity Index (R(2) = 3.1%), and Chronic Disease Score (R(2) = 1.7). CONCLUSIONS: Understanding how to efficiently predict these adverse outcomes in hospitalized elders is important for a variety of clinical and policy reasons. GIC and Cumulative Illness Rating Scale-Geriatrics may improve hospital discharge planning in a geriatric hospital treating very old patients with acute disease.