The natural history of hypertrophic cardiomyopathy is characterized by a slow progression of symptoms and by sudden death. Ventricular tachycardia is detected during electrocardiographic monitoring in 20 to 30% of patients; this arrhythmia is a sensitive but non-specific marker of those patients who are at particular risk of sudden death. Other specific prognostic features (e.g. anatomic and haemodynamic) to which preventive therapy could be directed remain to be identified. Episodes of supraventricular tachycardia or paroxysmal atrial fibrillation occur in 40-50% of patients and may cause troublesome symptoms. Propranolol, verapamil and conventional antiarrhythmic agents do not suppress serious ventricular arrhythmias or significantly reduce the incidence of paroxysmal supraventricular arrhythmias. The benzfuran derivative amioradone was used in 24 patients with refractory arrhythmias. Ventricular tachycardia was controlled in 18 of 19, paroxysmal supraventricular arrhythmias were abolished and sinus rhythm was restored in four of eight patients with long-standing atrial fibrillation. These results were maintained for at least 1 year. Longer observation is required to determine if control of arrhythmia with amiodarone will improve prognosis.