β-Blocker therapy is one of the principal therapies for congenital long-QT syndrome (LQTS). However, breakthrough cardiac events occur while being treated with β-blockers. We sought to determine the frequency of and clinical correlates underlying β-blocker therapy failures in genotyped, symptomatic LQTS probands. The medical records were analyzed only for genotyped LQTS probands who presented with a LQTS-attributable clinical event and were receiving β-blocker therapy. The study cohort comprised 28 such patients: 18 KCNQ1/KVLQT1(LQT1), 7 KCNH2/HERG (LQT2), and 3 SCN5A (LQT3). The prescribed β-blocker was atenolol (12), propranolol (10), metoprolol (4), and nadolol (2). β-Blocker therapy failure was defined as breakthrough cardiac events including syncope, aborted cardiac arrest (ACA), appropriate implantable cardioverter-defibrillator (ICD) therapy, or sudden death occurring while on β-blocker therapy. During a median follow-up of 46 months, 7/28 (25%) LQTS probands experienced a total of 15 breakthrough cardiac events. Their initial presentation was ACA (3), bradycardia during infancy (2), and syncope (2). The underlying genotype was KVLQT1 (6) and HERG (1). Two breakthroughs were attributed to noncompliance. Of the 13 breakthroughs occurring while compliant, 10 occurred with atenolol and 3 with propranolol (p = 0.03). In this study cohort, one-fourth of genotyped LQTS probands failed β-blocker therapy. Treatment with atenolol, young age at diagnosis, initial presentation with ACA, KVLQT1 genotype, and noncompliance may be important factors underlying β-blocker therapy failures.