Atrial fibrillation is the commonest arrhythmia observed in hypertrophic cardiomyopathy, and is associated with an acute deterioration in symptoms. Digoxin is the drug of choice in established atrial fibrillation and amiodarone the drug of choice in paroxysmal atrial fibrillation and ventricular arrhythmia. Non-sustained ventricular tachycardia occurs in 20% of patients and is the single best predictor of sudden death in adults. Sustained monomorphic ventricular tachycardia occurs only rarely. The mechanism of sudden death is likely to involve initiating factors such as arrhythmia and peripheral autonomic dysfunction causing haemodynamic instability and myocardial ischaemia. Myocardial disarray may provide the arrhythmogenic substrate such that haemodynamic instability and ischaemia results in ventricular fibrillation and sudden death.