Background: The effects of thyroid hormones on the cardiovascular system have been well documented. Chronotropic response and normal tone of the heart muscle during diastole are due to T3. Moreover, triiodothyronine affects the number of B adrenergic receptors and their sensitivity to catecholamines. Increased corrected QT (QTc) dispersion has been found to be associated with cardiac arrhythmias and sudden cardiac death in patients with myocardial infarction, left ventricular hypertrophy, congestive heart failure, diabetes and end-stage renal disease. Objective: The objectiveof this study was to evaluate QTc dispersion in children with congenital hypothyroidism. Patients and Methods: This was a case-control study that was carried out at Pediatric Cardiology and Endocrinology Units, Zagazig University Children's Hospital. The study included 74 children with hypothyroidism and control subjects. Twelve lead ECG was performed to all participants. Serum T4 and TSH were measured to all study members. Results: There was statistically significant increase among case than control groups regarding QTc dispersion (P < 0.05), while there was no statistically significant difference (P ≥ 0.05) regarding longest QTc, shortest QTc and QRS amplitude. The current study showed that, there was no statistically significant difference (P ≥ 0.05) regarding longest QT, shortest QT, QRS amplitude and QTc dispersion in cases with low versus those with high TSH level. There was no significant correlation between TSH and QTc dispersion. Conclusion:     QTc dispersion was higher in our patients compared to control group, which indicates heterogeneity of ventricular repolarization that could contribute to increased risk of ventricular arrhythmias.