This study sought to compare the efficacy and safety of intravenous flecainide and sotalol for immediate cardioversion of atrial fibrillation. We performed a prospective, randomized, single-blind, multicenter trial, including 106 hemodynamically stable patients with atrial fibrillation, stratified according to duration of the arrhythmia. Exclusion criteria included severely reduced left ventricular systolic function, recent antiarrhythmic therapy, and hypokalemia. Patients were randomly assigned to receive either intravenous flecainide or intravenous sotalol. Trial medication was given at a dose of 1.5 mg/kg body weight (maximum 150 mg). Overall, 28 of 54 patients (52%) given flecainide and 12 of 52 patients (23%) given sotalol converted to sinus rhythm during the first 2 hours after start of the infusion (p = 0.003). Multivariate analysis confirmed that treatment allocation to flecainide, an arrhythmia duration of < or = 24 hours, higher plasma magnesium level at baseline, higher age for men, and lower age for women independently increases the probability of conversion. The frequency of adverse effects was not significantly different in the 2 treatment groups.