Background: Over the past three decades, accumulating evidence has documented that pre-existing welldeveloped CCC at the onset of acute myocardial infarction plays an important role in preserving left ventricular function, reducing infarct size, preventing left ventricular aneurysm formation, and survival. Objective: The aim of this study is to evaluate the effect of collateral circulation as measured by Rentrop score on left ventricular regional and global systolic function in patients with totally occluded vessel pre and post PCI and the ensuing impact of successful PCI. Methodology: Sixty patients have single vessel coronary chronic total occlusion with viable myocardium of CTO related territories, successful revascularization at ERMED and ICS, between June 2017 and December 2018, were considered to participate in this prospective study, The study subjects were divided into three groups according to Rentrop score , each group contain 20 patients : Group (I) Rentrop score 0 , Group (II) Rentrop score 1 and Group (III) Rentrop score 2. Results: There was no statistically significant difference between the three groups regarding the incidence of diabetes, smoking and hypertension. There was high statistically significant difference in EF between each group after revascularization, EF was higher in group (III) than group (I) and group (II). There was statistically significant difference in WMSI comparison between each group after revascularization WMSI was lower in group (III) than group (I) and group (II). Conclusion: In general increase Rentrop score before revascularization lead to more improvement in WMSI and EF after successful revascularization with improvement of quality of life.