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Introduction: Implantable Cardioverter Defibrillators (ICD) are an important treatment option for Long QT syndrome (LQTS) patients at high risk of cardiac arrest. The aim of the study was to assess the effect of ICDs in reducing mortality in the overall LQTS population and in subgroups defined by ICD indications. Methods: This study included 3035 subjects (597 ever had an ICD) from the Rochester-based LQTS Registry with a QTc ≥ 470 ms or a confirmed LQTS mutation. Using multivariable Cox models with follow-up starting from 1 year of age, we assessed the relative risk of overall mortality, mortality censored at 50 years of age, and sudden cardiac death (SCD) associated with time-varying ICD status. The effect of ICD on the risk of mortality by subgroups (prior aborted cardiac arrest [ACA], prior syncope while on beta blockers, and all others without these two indications) was assessed by including interaction terms of the time-varying ICD status and the time-varying indication groups. Results: There were 389 deaths including 116 SCDs during a follow-up of 118,837 person-years. Those with an ICD had a 46% lower risk of death than those without an ICD (HR=0.54, 95%CI: 0.33 - 0.86), adjusted for other risk factors. The ICD HRs for mortality censored at 50 years of age and SCD were 0.25 (95%CI: 0.11 - 0.56) and 0.17 (95%CI: 0.06 - 0.50), respectively. Among those with prior ACA, ICD patients had an 82% lower risk of death than non-ICD patients (HR=0.18, 95%CI: 0.07 - 0.46). Among those with prior syncope on beta-blockers, ICD patients had a 57% (approaching statistical significance) lower risk of death (HR=0.43, 95%CI: 0.18 - 1.08) than non-ICD patients. Among other patients with no prior ACA nor syncope on beta-blockers, there was no evidence of a mortality benefit of ICD (Figure 1). Conclusions: ICD was associated with reduced risk of mortality and SCD in LQTS patients. The results by time-varying indication groups support the current recommendation regarding ICD implantation in LQTS patients. [ABSTRACT FROM AUTHOR]