Long QT syndrome type I is an autosomal dominant condition caused by the heterozygotic loss of the KCNQ1 gene function on the 11p15 chromosome. The KCNQ1 gene is located on chromosome 11 in an area that has been the subject of genetic imprinting and is involved in another genetic condition, Beckwith-Wiedemann syndrome. The authors present the case of a girl with inherited type I long QT syndrome (the patient's mother and sister are affected) associated with Beckwith-Wiedemann syndrome by hypomethylation of the IC2 imprinting center. MS-MLPA showed hypomethylation of the KvDMR locus (IC2), and Sanger sequencing performed revealed a pathogenic mutational variant in the KCNQ1 gene. Not every carrier of the pathogenic mutational variant in the KCNQ1 gene and IC2 hypomethylation exhibits both genetic disorders, for reasons that are not fully explained. Early diagnosis, close multidisciplinary monitoring, and adequate treatment are critical to the patient's optimal development and good prognosis.