Objectives: The purpose of this study was to investigate the follow-up and treatment of the mutation-carrying relatives of a proband with an inherited arrhythmia syndrome. Background: The congenital long QT syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia (CPVT), and Brugada syndrome (BrS) are primary inherited arrhythmia syndromes that may cause syncope and sudden cardiac death in young individuals. After establishing the disease-causing deoxyribonucleic acid (DNA) mutation in probands, we actively conducted cascade screening to identify, most often asymptomatic, relatives who are also at risk of life-threatening arrhythmias. Methods: We retrospectively collected data from our cardiogenetics database and patient records and analyzed whether the identified carriers received prophylactic treatment. Results: From 1996 to 2008, 130 probands with a disease-causing mutation in one of the involved genes were identified, and 509 relatives tested positive for the disease-causing familial mutation. These subjects subsequently underwent cardiologic investigation (electrocardiography, exercise testing, Holter monitoring, ajmaline testing, echocardiography, where appropriate). After a mean follow-up of 69 ± 31 months (LQTS), 60 ± 19 months (CPVT), and 56 ± 21 months (BrS), treatment was initiated and ongoing in 65% (199 of 308), 71% (85 of 120), and 6% (5 of 81) of the relatives in the LQTS, CPVT, and BrS families, respectively. Eight carriers were lost to follow-up. Treatment included drug treatment (n = 249) or implantation of pacemakers (n = 26) or cardioverter-defibrillators (n = 14). All mutation carriers received lifestyle instructions and a list of drugs to be avoided. Conclusions: Cascade screening in families with LQTS, BrS, or CPVT, which was based on DNA mutation carrying and subsequent cardiologic investigation, resulted in immediate prophylactic treatment in a substantial proportion of carriers, although these proportions varied significantly between the different diseases. © 2010 American College of Cardiology Foundation.