AbstractLong QT syndromes (LQTSs) can lead to sudden cardiac death, yet these syndromes are often asymptomatic and clinically undetected, despite the prolongation of the QT interval. Currently, when a disease-causing variant is identified in an individual, presymptomatic genetic testing is available and can form part of the recommended cascade testing to identify other family members at risk. We herein report the cases of two daughters who received presymptomatic genetic testing in infancy when the proband mother had been diagnosed with LQTS type 2 (LQT2; c.1171C > T, p.Q391X in KCNH2) after suffering from cardiac arrhythmia at 30 years of age. The daughters had a normal QTc interval, but they carried the same disease-causing variant as their mother. Children with family members who have genetically confirmed LQTS have a high risk of suffering from cardiac events later in life, so genetic testing is required. This poses a complex problem, as guidelines for medical intervention and follow-up systems among asymptomatic children with LQTS have yet to be established. Genetic testing should only be performed after adequate counseling to support children later in life. Individualized long-term genetic counseling is required for both parents and children at stages throughout life.