QT interval dispersion is an attempt to measure noninvasively the propensity of the heart to support reentrant circuits. QT dispersion increases in patients with acute myocardial infarction (MI) and a quicker restoration of blood flow in the infarct related artery decreases QT dispersion. Effects of collateral blood flow on QT dispersion and the occurence of ventricular arrhythmias is controversial. This study addresses the relationship between collateral blood flow, QT dispersion and ventricular arrhythmias. Eighty-two patients admitted within 6 hours and underwent thrombolytic treatment due to acute anterior MI were enrolled in this study. Twenty-five patients with collaterals were compared with age and genderly matched fifty-seven patients without coronary collaterals. Maximum corrected QT interval (QTc max) and corrected QT (QTc) dispersion values were higher in patients without collaterals both on admission and on the 5th day post-MI than those with collaterals. Ventricular arrhythmias were also more common in the patients without collaterals during hospitalization. QTc max on the fifth day post-MI was positively correlated with age, QTc dispersion was positively correlated with age and degree of LAD stenosis. Logistic regression analysis showed that only the collaterals and QT dispersion values affect the development of ventricular arrhythmias. In the patients with acute MI, collateral formation to the infarct related artery leads to decrease in QTc max, QTc disp values. Furthermore, these patients with collaterals had much lower arrhythmic events. All these findings support that the collateralisation at the time of infarct will reduce QTc dispersion and risk of re-entrant arrhythmia. Finally, we suggest that the collateral formation has a protective role on myocardial electrophysiology.