Background: Atrial fibrillation is common in older adults with heart failure. It is known to adversely affect outcomes. Aim: To examine the associations of atrial fibrillation with 4-year mortality and 30-day readmission in older adults hospitalized with heart failure. Methods: Patients were Medicare beneficiaries 65 years of age and older discharged with a primary diagnosis of heart failure. Baseline data were obtained by retrospective chart reviews and data on mortality and readmission were obtained from Medicare administrative files. Presence of atrial fibrillation was confirmed using electrocardiogram during hospital admission. Using Cox proportional hazards models we estimated bivariate and multivariable (adjusted for various patient and care covariates) hazards ratios (HR) and 95% confidence intervals (CI) for 4-year mortality and 30-day readmission of patients with atrial fibrillation compared with those without. Results: Patients (n=944) had a mean age (±S.D.) of 79 (±7) years, 61% were women, 18% African-Americans, 25% had atrial fibrillation by admission electrocardiogram, 64% died within 4 years, and 8% were readmitted. Patients with atrial fibrillation had a 52% increased risk of 4-year mortality (adjusted HR=1.52; 95%CI=1.11-2.07). Atrial fibrillation was also associated with higher risk of readmission (unadjusted HR=1.64; 95%CI=1.01-2.68). However, the association lost its statistical significance after adjustment for various patient and care variables (adjusted HR=2.09; 95%CI=0.94-4.65). Conclusion: Presence of atrial fibrillation was associated with significant increased risk of long-term mortality in older adults hospitalized with heart failure and was associated with a non-significant higher risk of hospital readmission. © 2003 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.