BACKGROUND: Chronic total occlusion (CTO) often triggers collateral coronary development, yet the relationship between angiographically graded collaterals and myocardial viability remains incompletely understood. We evaluated whether Rentrop collateral grading predicts tissue-level viability using contemporary quantitative cardiac magnetic resonance (CMR). METHODS: We enrolled 56 patients with angiographically confirmed CTO who underwent CMR. Collateral flow was graded using the Rentrop classification. Viable myocardium was defined as ≤50% transmural scar on late gadolinium enhancement. Quantitative myocardial blood flow (MBF) and scar burden were assessed within CTO territories. RESULTS: Higher Rentrop grades were linked to greater myocardial viability (Rentrop 0: 44.4% viable; Rentrop 2: 88.9% viable; p=0.043). CTO-territory stress MBF differed across Rentrop grades (p=0.04). In multivariable analysis, Rentrop grade emerged as the only independent predictor of viability (OR 2.52, 95% CI 1.18 to 5.42, p=0.01). Receiver operating characteristic analysis showed an area under the curve of 0.71, identifying Rentrop >1 as the optimal cut-off for predicting viability (sensitivity 66%, specificity 73%, p=0.01). In addition, there was a significant association between Canadian Cardiovascular Society anginal severity and collateral development (χ²=8.548, p=0.035). CONCLUSIONS: Angiographic grading of collaterals using the Rentrop score independently predicts myocardial viability in patients with CTO. These findings support integrating collateral assessment into risk stratification and revascularisation planning, particularly when advanced imaging is limited.