Background - Although atrial fibrillation (AF) is a common complication of acute myocardial infarction (MI), patient characteristics and association with outcomes remain poorly defined in the elderly. Methods and Results - We evaluated 106 780 Medicare beneficiaries ≥65 years of age from the Cooperative Cardiovascular Project treated for acute MI between January 1994 and February 1996 to determine the prevalence and prognostic significance of AF complicating acute MI in elderly patients. Patients were categorized on the basis of the presence of AF, and those with AF were further subdivided by time of AF (present on arrival versus developing during hospitalization). AF and non-AF patients were compared by univariate analysis, and logistic regression modeling was used to identify clinical predictors of AF. The influence of AF on outcomes was evaluated by unadjusted Kaplan-Meier survival curves and logistic regression models. AF was documented in 23 565 patients (22.1%): 11 510 presented with AF and 12 055 developed AF during hospitalization. AF patients were older, had more advanced heart failure, and were more likely to have had a prior MI and undergone coronary revascularization. AF patients had poorer outcomes, including higher in- hospital (25.3% versus 16.0%), 30-day (29.3% versus 19.1%), and 1-year (48.3% versus 32.7%) mortality. AF remained an independent predictor of in- hospital (odds ratio [OR], 1.21), 30-day (OR, 1.20), and 1-year (OR, 1.34) mortality after multivariate adjustment. Patients developing AF during hospitalization had a worse prognosis than patients who presented with AF. Conclusions - AF is a common complication of acute MI in elderly patients and independently influences mortality, particularly when it develops during hospitalization.