The efficacy and safety of oral flecainide for treatment of ventricular arrhythmias were assessed during a 3-day period in patients with various cardiac diseases. Of 11 patients who received a low dose of flecainide (median daily dose 240 mg), only 4 responded with 90% or greater reduction in premature ventricular complex frequency. Ventricular tachycardia could not be suppressed. During treatment no electrocardiographic changes occurred. 14 of the 19 patients who received a high dose of flecainide (median daily dose 480 mg), demonstrated a 90% or greater reduction in premature ventricular complexes, and ventricular tachycardia did not recur during treatment in 7 out of 9 patients. However, PQ, QRS, and QTc intervals were significantly increased. In general, flecainide was well tolerated and drug administration did not have to be discontinued because of side effects. Flecainide acetate treatment, with a median dose of 480 mg daily, appears to be highly effective for suppressing complex ventricular arrhythmias.