The effect of the new antiarrhythmic drug Flecainide was examined in a controlled long-term study (1 to 44 months [x = 17.7 +/- 12.4]) in 36 patients, aged 18 to 76 years (x = 44 +/- 16.3), suffering from ventricular arrhythmias. In 12 cases coronary heart disease, in 11 cases myocarditis, in 2 cases each of dilatative cardiomyopathy and mitral valve prolapse syndrome, and one in case each of combined aortic and mitral valve disease and postoperative condition of Fallot Trilogy was present. In 7 cases the etiology of the dysrhythmias could not be elucidated. 18 patients had been treated before by more than 3 other antiarrhythmic drugs without sufficient result. The daily dose administered was assessed by the degree of the dysrhythmias and the response to the drug. In most cases 300-400 mg Flecainide was given. The therapeutic success was assessed by 24 h Holter-ECG before and during therapy. In most cases two registrations were performed before and three registrations during therapy. In 29 patients (81%) a rate reduction of VES over 70% could be observed. Rate reduction of VT (n = 8) was total in 5 cases, in one case over 90%, in two cases over 70%. Salvos (n = 10) were abolished in 7 cases and reduced over 90% in two and over 70% in one of the cases. R-on-T-phenomena (n = 2) disappeared completely. An increase of rate or degree of the dysrhythmias was never observed. In no case had therapy to be interrupted due to severe side effects. Changes of laboratory values were not observed.(ABSTRACT TRUNCATED AT 250 WORDS)