In a recent clinical trial, the class Ic antiarrhythmic drugs encainide and flecainide were found to be associated with an increased mortality risk in patients with new myocardial infarction and ventricular arrhythmias. The purpose of this study was to assess whether an increased mortality risk also accompanied the use of these drugs to treat patients with supraventricular arrhythmias. Data were obtained from the respective pharmaceutical sponsors on the mortality observed with each drug in United States and foreign protocols enrolling patients with supraventricular arrhythmias. Mortality in the encainide population (343 patients) and the flecainide population (236 patients) was compared with that in a research arrhythmia clinic, the Duke population (154 patients). Nine deaths occurred in the combined encainide-flecainide population and 10 deaths occurred in the Duke population; the follow-up periods averaged 488 days and 1,285 days, respectively. The 6-year survival functions of these 2 populations, estimated by the Kaplan-Meier technique, did not differ significantly (p = 0.62). The hazard ratio for the combined encainide-flecainide population relative to the Duke population was estimated to be 0.6 with a 95% confidence interval of 0.2, 1.7. These descriptive comparisons did not demonstrate any excess mortality when flecainide and encainide were used in patients with supraventricular arrhythmias.