Cardiovascular disease is a leading cause of death worldwide. A number of these deaths are due to severe disturbances in cardiac rhythm (ventricular tachycardia or fibrillation) that lead to a fatal cardiac arrhythmia or sudden cardiac death. Ventricular tachycardia or fibrillation can occur during exercise or excitement, and has been associated with an underlying disorder called Long QT Syndrome (LQTS). The clinical course of LQTS is variable, some patients remain asymptomatic while others develop ventricular tachycardia or fibrillation, resulting in sudden death. LQTS is characterised by an abnormally long heart-beat length (QT interval) by ECG. The QT interval corrected for heart rate (QTc) is variable in the general population and has been shown to be highly predictive of a person’s risk of ventricular tachycardia or fibrillation and sudden cardiac death. ~75% of all LQTS cases have mutations in genes regulating QTc. However, there is large variation in the penetrance of these mutations on QTc, even within single affected families. Recent studies have shown that common genetic variants (SNPs) in genes coding for ionic channels, or neuronal nitric oxide synthase 1 regulator, NOS1AP, can affect QTc in the normal healthy population. Therefore SNPs within LQTS loci may modulate the effect of known mutations in LQTS patients. We will study SNPs in a large cohort of LQTS families with known mutations and predict that SNPs which prolong QTc are associated with a higher risk of cardiac events in the probands compared to asymptomatic LQTS patients. We will confirm the link between QTc and cardiac events in a large population, and that genotype is a determining factor for the age of occurrence of the first cardiac event. This research will help to explain the partial penetrance and high variability of QTc and risk of sudden death in LQTS families, and will have important implications for the early identification and prevention of arrhythmias and sudden cardiac death.