In a single-blind randomized study, the efficacy of intravenous flecainide (2 mg/kg/10 minutes) versus verapamil (10 mg/1 minute) was assessed in 40 patients with paroxysmal atrial fibrillation (AF) or atrial flutter (AFI). The treatment was considered successful if sinus rhythm occurred within 1 hour. Of 20 patients receiving flecainide, 14 of 17 (82%) with AF converted to sinus rhythm, but in 3 patients with AFI flecainide failed. All patients treated with verapamil (17 AF, 3 AFI) showed lower ventricular rates after 1 hour; however, only 1 (6%) with AF converted to sinus rhythm and 1 (6%) converted to AFI. Patients who did not convert to sinus rhythm after treatment with verapamil were treated with flecainide and observed for another hour. After the change to flecainide, 9 of 15 patients (60%) with AF still converted. Thus, 23 of 32 patients (72%) with AF and none of 7 with AFI converted to sinus rhythm after treatment with flecainide. Conversion to sinus rhythm was achieved in 19 of 22 patients (86%) when AF lasted less than 24 hours and in 4 of 10 (40%) when the arrhythmia lasted greater than 24 hours. Transient adverse effects were noted in 10 patients (26%) after flecainide. In summary, flecainide is an effective and safe drug for conversion of paroxysmal AF to sinus rhythm, but ineffective for AFI. Verapamil appears to be of no use for conversion of AF or AFI to sinus rhythm.