BACKGROUND- Homozygous or compound heterozygous mutations in KCNQ1 cause Jervell and Lange-Nielsen syndrome, a rare, autosomal-recessive form of long-QT syndrome characterized by deafness, marked QT prolongation, and a high risk of sudden death. However, it is not understood why some individuals with mutations on both KCNQ1 alleles present without deafness. In this study, we sought to determine the prevalence and genetic determinants of this phenomenon in a large referral population of patients with long-QT syndrome. METHODS AND RESULTS- A retrospective analysis of all patients with long-QT syndrome evaluated from July 1998 to April 2012 was used to identify those with ≥1 KCNQ1 mutation. Of the 249 KCNQ1-positive patients identified, 15 (6.0%) harbored a rare putative pathogenic mutation on both KCNQ1 alleles. Surprisingly, 11 of these patients (73%) presented without the sensorineural deafness associated with Jervell and Lange-Nielsen syndrome. The degree of QT-interval prolongation and the number of breakthrough cardiac events were similar between patients with and without deafness. Interestingly, truncating mutations were more prevalent in patients with Jervell and Lange-Nielsen syndrome (79%) than in nondeaf patients (36%; P<0.001) derived from this study and those in the literature. CONCLUSIONS- In this study, we provide evidence that the recessive inheritance of a severe long-QT syndrome type 1 phenotype in the absence of an auditory phenotype may represent a more common pattern of long-QT syndrome inheritance than previously anticipated and that these cases should be treated as a higher-risk long-QT syndrome subset similar to their Jervell and Lange-Nielsen syndrome counterparts. Furthermore, mutation type may serve as a genetic determinant of deafness, but not cardiac expressivity, in individuals harboring ≥1 KCNQ1 mutation on each allele.