ESVEM (Electrophysiologic Study Versus Electrocardiographic Monitoring) is an ongoing multicenter trial supported by the National Heart, Lung, and Blood Institute that began enrollment of patients on October 1, 1985. We describe here the methodology of the trial and data regarding enrollment of patients in the trial. The purpose of the trial is to determine whether electrophysiologic study or electroardiographic Holter monitoring more accurately predicts antiarrhythmic drug efficacy in patients with aborted sudden death or sustained ventricular tachycarrrhythmias. Consenting patients with inducible, sustained ventricular tachyarrhythmias and at least 480 premature ventricular contractions during 48 hours are randomized to undergo antiarrhythmic drug selection either by electrophysiologic study or by Holter monitoring. Up to six drugs (mexiletine, pirmenol, procainamide, propafenone, quinidine, and sotalol) are assessed in random order until one is predicted effective. An efficacy prediction is achieved in the electrophysiology limb if ventricular tachyarrhythmias are no longer inducible and in the Holter limb if ventricular ectopy is largely suppressed. Patients in whom a drug is predicted effective are followed while they are taking that drug to detect one of the three primary endpoints: arrhythmia recurrence, sudden death, or unmonitored syncope. In the first 37 months, 967 patients satisfied inclusion and exclusion criteria to undergo baseline studies. Two hundred eighty-six were eligible for and consented to randomization. In total, approximately 500 patients will be randomized and 285 subjects will be followed while receiving drugs that are predicted effective in this trial. Approximately 70 patients are expected to attain a primary endpoint during a mean follow-up of 3 years. The study will be capable of detecting a ratio in the median time to arrhythmia recurrence of the two groups of 1.6 or greater with a power ≥ 0.80 and a p ≤ 0.05.