Juvenile idiopathic arthritis (JIA) is the commonest cause of chronic inflammatory arthritis in childhood. Cardiac involvement as pericarditis, myocarditis and valvular disease is known to occur in patients with JIA (JIA), as it does in adults with rheumatoid arthritis. There are, however, few descriptions concerning systolic and diastolic functions of the left ventricle (LV) in children with JIA. QT dispersion (QTd) is simple noninvasive arrhythmogenic marker that can be used to assess homogeneity of cardiac repolarization and which has not been studied in JIA patients before. A recent study found that rheumatoid arthritis patients had an abnormally longer QTd and corrected QT (cQTd) dispersion, markers for ventricular arrhythmogenicity. This study assessed QTd and cQTd and their relation with systolic and diastolic function of the LV in a group of children with JIA. We performed electrocardiography and Doppler echocardiography on patients and controls. Maximum QT (QTmax), minimum QT (QTmin), QTd, corrected QT, maximum corrected QT (cQTmax), minimum corrected QT (cQTmin) and cQTd intervals were measured from standard 12-lead electrocardiography. No statistically significant differences were found between the groups in QTd and cQTd. Among the diastolic parameters, increased late flow velocity, decreased early flow velocity and prolonged isovolumic relaxation time reflected an abnormal relaxation form of diastolic dysfunction. During 12 months of follow-up, no ventricular arrhythmias were documented in either group.