Oral antiarrhythmic treatment with mexiletine and disopyramide was evaluated in 34 patients with recurrent sustained ventricular tachycardias by programmed ventricular stimulation, except in a few instances where spontaneous attacks occurred under therapy. Coronary heart disease was present in 17 patients, cardiomyopathy in 11, myocarditis in five, and mitral valve prolapse in one. Complete suppression of ventricular tachycardia was observed in three of 30 patients under mexiletine and in one of 25 patients under disopyramide. Disopyramide slowed the rate of the ventricular tachycardia considerably, while mexiletine had no such influence. For a mean of 24 months, 19 patients were maintained on either substance. Complete suppression of ventricular tachycardia during programmed stimulation predicted freedom from recurrences. Ventricular tachycardias recurred less frequently and at a slower rate in the other patients, but 31% have died. This study shows that complete suppression of ventricular tachycardia by mexiletine or disopyramide can be achieved only in a minority of patients with previously drug-resistant tachycardias.