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Although QT variables such as its interval and/or dispersion can be clinical markers of ventricular tachyarrhythmia, few data exist regarding the role of QT variables in genotyped hypertrophic cardiomyopathy (HCM). Therefore, we analyzed QT variables in genotyped subjects with or without left ventricular hypertrophy (LVH).QT variables were analyzed in 111 mutation and 43 non-mutation carriers who were divided into three groups: A, those without ECG abnormalities and echocardiographically determined LVH (wall thickness ≥13 mm); B, those with ECG abnormalities but LVH; and C, those with ECG abnormalities and LVH. We also examined clinical outcome of enrolled patients.Maximal LV wall thickness in group C (19.0 ± 4.3 mm, mean ±SD) was significantly greater than that in group A (9.2 ± 1.8) and group B (10.4 ± 1.8). Under these conditions, maximum QTc interval and QT dispersion were significantly longer in group C than those in group A (438 ± 38 ms vs 406 ± 30 and 64 ± 31 vs 44 ± 18, respectively; P < 0.05). QTc interval and QT dispersion in group B (436 ± 50 and 64 ± 22 ms) were also significantly greater than those in group A. During follow-up periods, four sudden cardiac deaths and one ventricular fibrillation were observed in group C, and two nonlethal ventricular tachyarrhythmias were observed in group B.Patients with HCM-related gene mutation accompanying any ECG abnormalities frequently exhibited impaired QT variables even without LVH. We suggest that careful observation should be considered for those genotyped subjects.