The efficacy of flecainide acetate for conversion of atrial fibrillation into sinus rhythm was assessed in 69 patients (mean age of 63 +/- 14 years). Mean duration of the arrhythmia was 49 +/- 45 days. Mean cardiothoracic index was 0.49 +/- 0.03. Flecainide treatment was started intravenously with a bolus of 2 mg/kg over 10 minutes, followed by oral treatment (200 to 300 mg/day) according to body weight. Conversion to sinus rhythm was obtained in 49 patients (71%). The mean delay between initiation of treatment and restoration of sinus rhythm was 301 minutes (range 5 to 1,600). The left atrial diameter was smaller (40 +/- 1 mm) in patients who had successful cardioversion than in those who did not (46 +/- 1 mm) (p less than 0.05). Patients with atrial fibrillation lasting for less than 10 days had a higher conversion rate (79%) than patients with long-standing atrial fibrillation, in whom the conversion rate was only 38% (p less than 0.05). Conversion to sinus rhythm occurred in 33 patients during the first 5 minutes after injection. Adverse effects necessitated discontinuation of treatment in 4 patients (5.8%). Gastrointestinal disorders and dizziness occurred in 5 other patients but did not necessitate discontinuing treatment. In conclusion, flecainide is an effective drug for converting atrial fibrillation into sinus rhythm. Unlike quinidine, flecainide can be administered intravenously. The conversion rate with flecainide is higher in patients with a shorter duration of atrial fibrillation and smaller atria.