Sudden death is a tragic complication of hypertrophic cardiomyopathy. We report the case of a young patient with hypertrophic cardiomyopathy in whom an episode of atrial fibrillation triggered ventricular fibrillation and cardiac arrest. A 21-year-old man with nonobstructive hypertrophic cardiomyopathy underwent cardioverter-defibrillator implantation for primary prevention of sudden death, after risk stratification with noninvasive strategies. After 6 weeks, during a moderate effort, the patient had a syncopal episode, preceded by palpitations and dizziness, and terminated by the cardioverter-defibrillator. Device interrogation revealed an episode of atrial fibrillation with high ventricular response, spontaneously followed by ventricular tachycardia/fibrillation. Atrial fibrillation is a potential trigger of life-threatening arrhythmias and sudden death in patients with hypertrophic cardiomyopathy. Clinical investigation of risk markers for sudden death should be encouraged to identify high-risk patients who may benefit from a prophylactic therapy with an implantable cardioverter-defibrillator.