Thirty-nine patients with symptomatic ectopic atrial tachycardia (9 paroxysmal, of which 5 were incessant) and atrial fibrillation (AF) (25 paroxysmal, 5 chronic) were treated with oral flecainide acetate (100 to 400 mg/day). Thirty-two patients had organic heart disease (16 coronary artery disease, 6 valvular, 10 cardiomyopathy, 7 primary electrical abnormality). Previous antiarrhythmic trials consisted of 0 to 5 drugs (mean 2.2). Of 39 patients with atrial tachycardia or AF, a complete response (no recurrent symptomatic atrial arrhythmia) was achieved in 22 (56%), a partial response (more than 95% reduction in arrhythmia occurrence) in 3 (8%) and no response in 14 (36%). Left atrial size, ejection fraction, underlying heart disease, duration of symptoms before treatment and drug levels were not useful for predicting clinical response. Therefore, during the follow-up period of 5.4 +/- 6.7 months (range 4 weeks to 2.5 years), flecainide had a complete or partial effect in 25 patients (64%). Complete or partial responses were noted in 8 of 9 patients (90%) with ectopic atrial tachycardia and 17 of 30 (57%) with AF. In 14 patients with concurrent ventricular arrhythmias, a significant reduction in episodes of nonsustained ventricular tachycardia was also achieved. Treatment was discontinued in 8 patients (20%) because of cardiac adverse reactions, including pulmonary edema and ventricular or atrial proarrhythmic response. Thus, oral flecainide acetate is effective therapy for some patients with ectopic atrial tachycardia or AF.