Background Sudden cardiac death(SCD)is a major cause of mortality in adults with congenital heart disease(CHD).The aim of this study was to determine the adult CHD population at risk of SCD and the clinical parameters associated with SCD.Methods and Results We performed a multicenter case-controlled study.Patients who died suddenly due to proven or presumed arrhythmia were included(cases).For each case,two controls matched on diagnosis,type of surgical intervention,age and gender were included.From three databases including 25790 adults with CHD,1189 deaths(5%)were identified,of whom 213 patients(19%)died suddenly.Arrhythmic death occurred in 171 of 1189 patients.The underlying cardiac lesions were mild,moderate and severe CHD in 12%,33% and 55% of the SCD cases,respectively.Clinical variables associated with SCD were supraventricular tachycardia(SVT,OR 3.5,95% CI 1.5-7.9,P = 0.004),moderate to severe systemic ventricular dysfunction(OR 3.4,95% CI 1.1-10.4,P = 0.034),moderate to severe subpulmonary ventricular dysfunction(OR 3.4,95% CI 1.110.2,P = 0.030),increased QRS duration(OR 1.34(per 10 ms increase),95% CI 1.10-1.34,P = 0.008),and QT dispersion(OR 1.22(per 10 ms increase),95% CI 1.22-1.48,P = 0.008).Conclusions The clinical parameters found to be associated with SCD in adults with a broad spectrum of CHD,including systemic right ventricles,are similar to those in ischemic heart disease.Moreover,even those patients with mild cardiac lesions,are potentially at risk for SCD.This highlights the need for further prospective studies as well as vigilant ongoing follow-up of the adult with CHD. Background Sudden cardiac death (SCD) is a major cause of mortality in adults with congenital heart disease (CHD). The aim of this study was to determine the adult CHD population at risk of SCD and the clinical parameters associated with SCD. Methods and Results We performed a multicenter case-controlled study. Patients who died suddenly due to proven or presumed arrhythmia were included (cases) .For each case, two controls matched on diagnosis, type of surgical intervention, age and gender were included. Fc three databases including 25790 adults with CHD, 1189 deaths (5%) were identified, of whom 213 patients (19%) died suddenly. Arrhythmic death occurred in 171 of 1189 patients. The underlying cardiac lesions were mild, moderate and severe CHD in 12%, 33 % and 55% of the SCD cases, respectively. Clinical variables associated with SCD were supraventricular tachycardia (OR 3.5,95% CI 1.5-7.9, P = 0.004), moderate to severe systemic ventricular dysfunction (OR 3.4,95% CI 1.1-10.4, P = 0.034), moderate to severe sub-pulmonary heart dysfunction (OR 3.4,95% CI 1.110.2, P = 0.030), increased QRS duration (OR 1.34 per 10 ms increase), 95% CI 1.10-1.34, P = 0.008) (per 10 ms increase), 95% CI 1.22-1.48, P = 0.008) .Conclusions The clinical parameters found to be associated with SCD in adults with a broad spectrum of CHD, including systemic right ventricles, are similar to those in ischemic heart disease. More of those patients with mild cardiac lesions, are potentially at risk for SCD.This highlights the need for further prospective studies as well as vigilant ongoing follow-up of the adult with CHD.