Moricizine was studied in 908 patients with ventricular arrhythmia. Proarrhythmia occurred in 29 (3.2%). When the type of proarrhythmia and the type of ventricular arrhythmia were correlated, no proarrhythmic events occurred in patients with benign ventricular arrhythmia. Three of the 4 deaths due to proarrhythmia occurred in patients with lethal ventricular arrhythmia and 14 of the 15 serious proarrhythmic events occurred in patients with potentially lethal ventricular arrhythmia. The overall proarrhythmia incidence in lethal and potentially lethal ventricular arrhythmias was not different (3.2 vs 3.7%, respectively). Proarrhythmia occurred in patients with more significant structural heart disease or conduction defects at baseline, but was not related to the baseline frequency of ventricular premature complexes. There was no relation between dose of moricizine and incidence of proarrhythmia. All 29 proarrhythmic events occurred within 10 days and 26 of 29 (90%) took place within 7 days of therapy start. Thus, moricizine has a low proarrhythmic potential, especially in patients with lethal ventricular arrhythmias, and may have the best risk/benefit ratio among first-line drugs used in these patients.