In 69 consecutive patients with recent-onset atrial fibrillation, we compared the usefulness of verapamil, flecainide and propafenone for the acute conversion to sinus rhythm within 1 hour of the start of intravenous infusion. The patients were randomized to receive verapamil, 10 mg in 5 minutes (n = 29), flecainide, 2 mg/kg in 10 minutes (n = 20) or propafenone, 2 mg/kg in 10 minutes (n = 20). The 3 treatment groups did not differ significantly regarding patient baseline characteristics. Conversion rate was 14% for verapamil, 50% for flecainide and 25% for propafenone. A significant difference (p = 0.02) was found only between the verapamil and flecainide results. Mean time to conversion did not differ significantly. Stepwise multiple regression analysis revealed that the only variables correlating with the final result were the type of treatment (p = 0.001) and the duration of arrhythmia (p = 0.005). Mean duration of arrhythmia in converted patients was significantly shorter than in non-converted patients (9.7 [6.6] vs 34.0 [46.1] hours, p = 0.03). Serious side effects were seen only during propafenone infusion (2 patients developed severe hypotension necessitating discontinuation of therapy and administration of intravenous fluids). We conclude that intravenous flecainide appears to be more effective than the other 2 drugs for the acute conversion of recent-onset atrial fibrillation to sinus rhythm.