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We assessed the outcome of cascade screening of families with congenital long QT syndrome (LQTS) in Danish heart centers.Affected family members were identified through systematic family screening.In total, 228 affected relatives were identified from 90 families. A disease-causing mutation useful for presymptomatic genetic testing was found in 82% of probands. Two-thirds of affected relatives fulfilled electrocardiographic criteria for the diagnosis, whereas diagnosis was based on genetic findings in only one-third. The majority of affected relatives were asymptomatic. Symptomatic relatives and probands most often presented with syncope, followed by aborted cardiac arrest and sudden cardiac death. A serious cardiac event (SCE, such as syncope, aborted cardiac arrest or cardiac arrest) was reported by 32% of affected relatives and 87% of probands (p < 0.0001). Fifty-two percent of affected relatives were on β-blockers and 11% had an implantable cardioverter defibrillator (ICD), as compared to 88 and 49% of probands (p < 0.0001). Appropriate ICD therapy was given to 13% of affected relatives and to 27% of probands (p = 0.1).Clinically driven cascade screening of Danish LQTS families identified 2-3 affected relatives per proband. Affected relatives had milder disease courses, but SCEs in a subset strongly support screening. Danish cardiologists have adopted cascade screening of LQTS families according to specific Danish guidelines.