The safety of encainide has been evaluated using retrospective analyses of the Bristol-Myers Supraventricular and Ventricular Arrhythmias data base and of the Post-Marketing Adverse Experience Report data and prospective analyses of the Cardiac Arrhythmia Suppression Trial (CAST), the Cardiac Arrhythmia Pilot Study (CAPS), and the Ventricular Tachycardia/Heart Disease and Boston studies. CAST, a randomized, placebo-controlled study in patients with a history of myocardial infarction with asymptomatic or minimally symptomatic ventricular arrhythmias, showed that sudden death or nonfatal cardiac arrest occurred more frequently on encainide (24/418, 5.7%) than on placebo (7/416, 1.7%). The highest sudden death/cardiac arrest rates were found in patients with a left ventricular ejection fraction of less than 0.30, those with a ventricular premature beat count of more than 50/hr and those with a myocardial infarction of more than 90 days. Similar sudden death/cardiac arrest rates were seen in the flecainide-treated group of the study but not in the moricizine-treated group. A retrospective analysis of the data collected from a similar cohort of patients in the Bristol-Myers data base showed a 1-year cumulative incidence of 10.2% in patients with a history of myocardial infarction. A retrospective analysis of mortality data in patients with supraventricular arrhythmias (301 patients) showed this to be slightly lower than in a matched sample of the general U.S. population. The sudden death mortality in the Ventricular Tachycardia/Heart Disease and Boston studies were similar to those reported with other antiarrhythmic agents. Abnormal laboratory findings caused four patients to be discontinued prematurely, but there have been no reported cases of any blood dyscrasias. Thus, there are currently no data showing that patients with symptomatic reentry supraventricular and life-threatening ventricular arrhythmias are at increased risk with encainide therapy. Encainide should be reserved for those patients who are refractory or intolerant to other antiarrhythmic agents. Encainide is not indicated in patients with symptomatic ventricular arrhythmias and structural heart disease. In patients without structural heart disease and symptomatic ventricular arrhythmias, the benefit and risks of encainide therapy should be carefully considered before it is prescribed.