Background Although anticoagulation therapy is accepted for most patients with atrial fibrillation, 2 different strategies exist for management of the cardiac rhythm: atrial fibrillation is allowed to persist while the ventricular rate is controlled; and atrial fibrillation is converted, and an attempt is made to maintain sinus rhythm. Methods The Atrial Fibrillation Follow-up Investigation of Rhythm Management (AFFIRM) Study was a randomized clinical trial that compared these 2 strategies. We report the baseline characteristics of the patients enrolled in the AFFIRM Study. Results More than 7400 patients at more than 200 North American hospitals and clinics qualified for enrollment in the AFFIRM Study. A total of 4060 patients were enrolled in the AFFIRM Study. The average age of patients enrolled was 70 years, with 39% female and 89% white. Hypertension was present in 71%. Coronary artery disease was present in 38%. Echocardiography was performed in 3311 patients, and results showed normal ventricular function in 68% and normal left atrial size in 33%. Most patients with recurrent episodes had symptoms with atrial fibrillation. Approximately one third of patients were enrolled with a first episode of atrial fibrillation. Conclusion The AFFIRM Study enrolled 4060 predominantly elderly patients with atrial fibrillation to compare ventricular rate control with rhythm control. The patients in the AFFIRM Study were representative of patients at high risk for complications from atrial fibrillation, which indicates that the results of this large clinical trial will be relevant to patient care. (Am Heart J 2002;143:991-1001.)