Background and Objectives : Long QT syndrome (LQTS) is considered as an electrical disease caused by gene mutation. Compared to those of Brugada syndrome (BS), which is also considered as another kind of electrical disease, the clinical significance of late potential in patients with LQTS is not well known. Subjects and Methods : We performed signal-averaged ECG (SAECG) in 8 symptomatic patients (27짹14 years old, 4 male) with LQTS. All suffered from exercise- or exertion-related syncopes or documented ventricular tachyarrhythmias and were normal in chest X-ray and 2-D echocardiography. Routine criteria of late potentials with noise level <0.3 uV in patients with QRS duration <110 msec were considered as positive for late potentials. Results : The results of SAECG were compared with 14 patients (49짹14 years, all males) with patients with Brugada syndrome. Patients with LQTS were younger, more female, and showed shorter fQTS, LAS40, RMS40. Two patients (25%) with LQTS and 11 patients (78.6%) with BS were positive for late potentials (P=0.026). There are no significant correlations between QTc and SAECG variables in LQTS patients. And the correlations between BMI and SAECG variables are not significant either. Conclusion : Although the clinical significance of late potentials is unclear in patients with LQTS some patients are late potential positive. Further studies are needed.