In a group of 62 patients with chronic heart failure 29% developed ventricular tachycardia during exercise testing. The two-year survival of this group was only 22%, compared to 55% in those not demonstrating ventricular tachycardia (P less than 0.01). In a double-blind cross-over study against placebo, with three-month treatment periods, amiodarone, in a dose of 200 mg day-1, was found effectively to suppress ventricular arrhythmias during exercise and in the immediate post-exercise period. Exercise performance was not impaired. Amiodarone may be useful in preventing life-threatening arrhythmias in patients with heart failure.