The long QT syndrome (LQTS) is a congenital disorder characterized by a prolongation of the QT interval on electrocardiogram and a propensity to ventricular tachyarrhythmias, which may lead to cardiac events defined as syncope, cardiac arrest, or sudden death. Children are very frequently affected by LQTS accounting for about 50% of probands and 40% to 50% affected family members enrolled in the International LQTS Registry. LQTS probands stratified by age 0 to 5 years, 6 to 10 years, and 11 to 15 years showed that QTc is longer in the youngest group only when using Bazett's heart rate correction. However, when using Rautaharju's or Karjalainen's corrections, which adjust better for higher heart rate than Bazett's correction does, this difference is no longer present. Gender influences the risk of cardiac events in LQTS children with boys having significantly higher risk than girls by age 15 years, despite similar magnitude of QT prolongation. Genotype also influences clinical course of LQTS with LQT1 and LQT2 carriers having higher risk than LQT3 carriers. The risk varies by age among 3 genetic types of LQTS: LQT1 carriers are at higher risk of cardiac events between age 5 to 15 years than below age of 5 years, LQT2 carriers have the highest risk of cardiac events at age 10 to 15, and LQT3 carriers have infrequent cardiac events below age of 10 years. This pattern is observed in both boy and girl LQTS children. In conclusion, there is substantial age, gender, and genotype effect on the clinical course of LQTS children indicating the need of adjusting for those factors in clinical practice.