Abstract Flecainide has been shown to be effective in short-term, controlled studies for prevention of paroxysmal supraventricular tachycardia (SVT) and paroxysmal atrial fibrillation (AF). However, it is unknown whether this beneficial response is maintained during long-term chronic therapy. Forty-nine patients were studied who enrolled in double-blind, placebo-controlled, short-term studies of safety and efficacy and subsequently received long-term, open-label therapy for ≥6 months (mean duration of therapy, 17 months). To evaluate chronic efficacy, events during long-term therapy were documented by a transtelephonic monitor for either 4 or 8 weeks, comparable to the corresponding 4- or 8-week placebo-baseline periods in the same patients. Results during chronic therapy were compared with those at baseline and after the initial (shortterm) treatment period. Compared with placebobaseline results, the number of patients free of arrhythmic attacks increased significantly for both patients with SVT (from 24% to 82%, p = 0.013, n = 17) and patients with AF (from 12% to 68%, p <0.001, n = 25). Mean time to first attack and mean number of days between attacks also showed significant and parallel increases during the chronic efficacy period. In patients with paired short- and long-term efficacy evaluations with the same dose of flecainide, end points were maintained at equivalent levels or showed further improvement (i.e., mean rate of AF attacks decreased further with chronic therapy, p = 0.036). No proarrhythmic events, death, or myocardial infarction occurred. In this select group of patients, oral flecainide therapy continued to be effective and safe in the long-term treatment of paroxysmal SVT and AF.