Flecainide acetate was administered to 19 patients who had inducible sustained orthodromic atrioventricular reentrant tachycardia. Eleven of 18 patients had no inducible tachycardia and 7 patients continued to have inducible tachycardia while receiving flecainide. The effects of flecainide could not be evaluated because of hypotension during intravenous infusion in 1 patient. The main effect of the drug was a selective depression of retrograde conduction over the bypass tract, resulting in abolition of reentry or prolongation of tachycardia cycle length. The electrophysiologic effects of intravenous flecainide were concordant with those of oral flecainide in 5 patients who were studied during both regimens. Fifteen patients were discharged from the hospital on a regimen of flecainide. One patient received concomitant beta-blocking therapy. During an average follow-up of 18.5 months (range 2 to 48), 9 patients remained symptom free. Recurrences were observed in 5 patients, 3 of whom still had inducible tachycardia during electrophysiologic testing. Treatment had to be discontinued in 1 patient because of drug intolerance. Thus, flecainide is likely to be effective and well tolerated in the long-term treatment of at least 50% of patients who present with bypass tract mediated orthodromic reentrant tachycardia.