BACKGROUND: Long QT is a cardiac electrical disorder. One of the symptoms of long QT caused by fatal ventricular arrhythmia is seizure. In some studies it was indicated that up to 35% of seizures induced by long QT may be misdiagnosed as other causes of seizure. METHODS: In a case-control study, patients experiencing primary seizure with unknown etiology and referring for clinical diagnosis were selected as the case group. The control group consisted of patients hospitalized for other reasons except seizure. Corrected QT Interval (QTc) was measured for each patient on an electrocardiogram, and the two groups were compared. Long QT was defined as a QT more than 0.46 seconds. RESULTS: Among 508 subjects who were recruited in this study 254 children were in the case group and 254 were in the control group. There were 66 children experiencing seizure and long QT in the case group. In the control group, 48 children with long QT were observed; the difference was statistically significant (P = 0.02). Syncope and sudden death were not significantly different between the two groups. CONCLUSION: The present study showed that children with unknown causes of seizure have more frequently long QTc, which implies the possibility of an arrhythmic origin of some seizures. Therefore, it is advised to get an electrocardiography for patients with unknown causes of seizure.   Keywords: Seizure, Children, Long QTc