Flecainide, a Class IC antiarrhythmic agent, was used in 12 patients with an average age of 57 years to treat spontaneous monomorphic sustained ventricular tachycardia (S-VT, n = 9), with a ventricular rhythm of 203 +/- 41 bpm (5 right bundle branch and 4 left bundle branch block pattern) and non-sustained ventricular tachycardia (NS-VT, n = 3). The patients had ischaemic heart disease (n = 5, including 2 cases of aneurysm), idiopathic dilated cardiomyopathy (n = 1), ventricular dysplasia (right, n = 1; left n = 2; biventricular, n = 1). The remaining 2 patients had no overt cardiac disease on coronary angiography. None of the patients had signs of cardiac failure; the left ventricular ejection fraction was 0.49 +/- 0.7. Before treatment, programmed ventricular stimulation (PVS) induced 12 S-VT (214 +/- 41 bpm) which reproduced the clinical VT in 8 out of 10 cases. A second series of electrophysiological studies was performed after an average of 5 weeks treatment with Flecainide 300 mg/day (200-400 mg). It was not possible to induce VT in 2 patients (17% total prevention); NS-VT replaced S-VT in 4 patients (33%); S-VT was less rapid in 5 patients (at least 50 bpm slower) (41%); one patient had S-VT as rapid as before treatment (9%). The 12 patients were prescribed long-term Flecainide therapy. During follow-up there were 4 early (7, 10 and 15 days) and one late recurrence (16 months) (42% failure rate) whilst the other 7 patients had no further attacks of VT (follow-up of 19.1 +/- 5 months) (58% success rate).(ABSTRACT TRUNCATED AT 250 WORDS)