AbstractBackground: The purpose of the present paper was to determine the clinical and electrophysiological features of Japanese pediatric long QT syndrome (LQTS) patients with KCNQ1 mutations (LQT1).Methods: KCNQ1 mutations were analyzed in 13 Japanese pediatric patients with prolonged QT interval on electrocardiogram (ECG). These LQT1 patients were reviewed, retrospectively, for the presence of past and family histories of LQTS‐related cardiac events, other complications, and their ECG findings evaluated at rest and during exercise).Results: KCNQ1 mutations were identified in eight patients (61.5%) from six unrelated families. Four missense mutations were identified in seven patients and an insertion/deletion in one. The mutations were located in the transmembrane domain in four patients and the C‐terminal domain in four. Syncope did not occur in patients with the C‐terminal domain mutations up to the age of 6–9 years, but family members of patients with the C‐terminal domain mutations had a history of syncope in their elementary school days. Compared with a non‐LQTS control group, peak heart rate (HR) on exercise and the HR increase during exercise were significantly lower in the LQT1 group (LQT1 vs control, 155 ± 16 beats/min vs 182 ± 13 beats/min, P < 0.01, 66 ± 16 beats/min vs 99 ± 24 beats/min, P < 0.01, respectively).Conclusions: The risk of LQTS‐related cardiac events may not be different in pediatric LQT1 patients with C‐terminal domain mutations than in patients with transmembrane domain mutations. Possible sinus node dysfunction or a poor HR response to sympathetic stimulation has been suggested in pediatric LQT1 patients.