The effectiveness and safety of flecainide and quinidine for conversion of atrial fibrillation (AF) to sinus rhythm were compared. Sixty consecutive patients were treated with either flecainide (up to 2 mg/kg intravenously and then orally) or quinidine (up to 1.2 g orally). There was no statistical difference in age, left atrial size, duration of the arrhythmia and underlying cardiac diseases between the 2 groups. The overall conversion rate to sinus rhythm was 63% (38 patients): AF was converted in 18 patients (60%) treated with quinidine and 20 (67%) with flecainide. If AF lasted less than 10 days, the conversion rate was 86% in the flecainide group and 80% in the quinidine group (difference not significant). When AF lasted more than 10 days the rate was 22% in the flecainide group and 40% in the quinidine group. Adverse effects were more frequent in the quinidine group (27%) (gastrointestinal disturbances) than in the flecainide group (7%) (conduction disturbances), but they were less severe in the quinidine group. Thus, flecainide given intravenously appeared to be as effective as quinidine given orally for conversion of AF of recent onset (within 10 days). However, quinidine should probably remain the preferred drug for conversion of AF of long duration (more than 10 days) to sinus rhythm. Adverse effects occurred less often with flecainide therapy, but they were more severe.