On the basis of epidemiologic studies, more than 10 million Americans have echocardiographic evidence of mitral valve prolapse. Although ventricular arrhythmias occur frequently (over 50 percent of patients with mitral valve prolapse), they rarely result in sustained ventricular tachycardia or sudden cardiac death. However, a common problem in clinical practice is a patient with mitral valve prolapse and symptomatic complex ventricular arrhythmias refractory or intolerant to both beta blockers and conventional type I antiarrhythmics. These drugs are known to have frequent side effects, toxicity, and proarrhythmic effects. In 17 patients with mitral valve prolapse who presented with symptomatic complex ventricular arrhythmias and who were unresponsive to an average of the three conventional agents, moricizine (Ethmozine) was effective in suppressing 90 percent of ventricular premature depolarizations, 99 percent of nonsustained runs of ventricular tachycardia, as well as all sustained runs of ventricular tachycardia, resulting in abolition of palpitations, dizziness, and syncopal episodes. Its efficacy as well as its low frequency of minor side effects makes it ideal for future consideration in the population with mitral valve prolapse, who are frequently young and may therefore require therapy for many years.