The effectiveness of intravenous propafenone for conversion to sinus rhythm (SR) of paroxysmal atrial fibrillation (AF), lasting less than 7 days, was evaluated with a single-blind, randomized, placebo-controlled study, given the possible spontaneous conversion of this arrhythmia. Group 1 (98 patients) received intravenous propafenone (2 mg/kg iv over 10 minutes followed by 0.007 mg/kg/min); and group 2 (84 patients) received intravenous placebo (0.9% saline solution). The infusion was continued until restoration of SR but no longer than 24 hours. Eight-nine patients (90.8%) received propafenone and 27 patients (32%) receiving placebo were converted to SR (p < 0.005). The mean conversion time was 2.46 +/- 2.59 hours in group 1 and 17.15 +/- 5.78 hours in group 2 (p < 0.005). In patients treated with propafenone, conversion of SR mostly occurred in the first 4 hours (86.5%), considered to be the optimal infusion time in our experience. In both groups, the left atrial size was significantly larger in nonconverted than in converted patients. Similarly, the duration of the arrhythmia was significantly longer in nonconverted patients. In nonconverted patients, the mean ventricular rate decreased from 143 +/- 16 beats/min to 101 +/- 18 beats/ min after propafenone and from 135 +/- 19 beats/min to 119 +/- 16 beats/min after placebo (group 1 vs. group 2: p < 0.005). Two episodes of sinus standstill (3.4 and 3.8, seconds, respectively) occurred at SR restoration obtained with propafenone. Intravenous propafenone is an effective, safe, and usually rapid drug for AF treatment. Moreover, it produces a real and significant reduction in the mean ventricular rate in nonconverted patients.