Acecainide (N-acetylprocainamide), the N-acetylated metabolite of procainamide, is a Class III antiarrhythmic agent. It can be given either intravenously or orally, and is eliminated primarily by renal excretion. In a small number of noncomparative and placebo-controlled short term therapeutic trials acecainide markedly reduced premature ventricular beats and prevented induction of ventricular tachycardia in more than 70% of patients following intravenous administration and in about 50% after oral administration. Acecainide was effective in about one-quarter of patients refractory to other antiarrhythmic drugs. Interpretation of its effectiveness following long term oral therapy is complicated by the limited number of patients, and patients discontinuing due to adverse effects or lack of efficacy. However, about 40% of the small number treated for extended periods were controlled for periods of 6 months to 3 to 4 years. Comparative studies with other antiarrhythmic drugs have not been undertaken apart from a small study in atrial flutter where acecainide was better than quinidine plus digoxin. Thus, although further clinical experience is required before the relative place of acecainide in therapy can be determined, the drug nevertheless appears to offer advantages over procainamide, particularly with respect to the reduced formation of antinuclear antibodies.