We sought to assess the risk of sudden cardiac death (SCD) and ventricular arrhythmia after alcohol septal ablation (ASA) for obstructive hypertrophic cardiomyopathy. ASA is a nonsurgical alternative to septal myectomy for treatment of symptomatic, drug-refractory, obstructive hypertrophic cardiomyopathy. The effect of ASA on ventricular arrhythmia risk is not well established. We examined the rates of SCD among 89 patients treated with ASA. The secondary end point was ventricular tachycardia/ventricular fibrillation (VT/VF), appropriate implantable cardioverter defibrillator (ICD) therapy, or cardiac arrest after ASA among those with implanted ICDs or permanent pacemakers (n = 42). Patients were classified as either high-risk or low-risk on the basis of established clinical indications for ICD implantation. No mortality was attributable to SCD at a mean follow-up of 5.0 +/- 2.3 years in the entire cohort. Among the 42 patients with an ICD or permanent pacemaker, 9 had documented VT/VF, cardiac arrest, or appropriate ICD therapy, resulting in an annual event rate of 4.9%/year. The annual event rate for VT/VF, cardiac arrest, or appropriate ICD therapy was 2.8%/year (4 of 29 patients) in low-risk patients and 13.4% in high-risk patients (5 of 13 patients). A 10-mm Hg increase in the immediate post-ASA gradient was associated with a hazard ratio of 2.66 for arrhythmic events (95% confidence interval 1.55 to 4.56, p <0.001). In conclusion, ASA was performed in patients with highly symptomatic, drug-refractory hypertrophic cardiomyopathy with no mortality attributable to SCD and an annual rate of VT/VF, cardiac arrest, or appropriate ICD therapy of 4.9%/year.