This review summarizes the data from all the studies conducted in the United States and Europe that have evaluated the efficacy of encainide in patients with a variety of supraventricular arrhythmias. Using clinical criteria of efficacy, encainide was found to be effective or partially effective in 77% of patients evaluated by electrophysiologic means. Similar levels of efficacy were observed in patients with AV as well as AV nodal reentry arrhythmias. Patients with other forms of supraventricular arrhythmias in smaller series also experienced comparable degrees of efficacy. Encainide was discontinued because of intolerance by 8% of the patients in these studies: 6% because of side effects and 2% because of proarrhythmia.