Background <p>Chronic total occlusion (CTO) is a challenging subset of coronary artery disease characterized by complete vessel obstruction and development of collateral circulation. In CTO patients, donor vessels supplying collateral flow may be at risk of functional ischemia. Functional assessment of these donor vessels can provide valuable insights into the ischemic status. Quantitative flow ratio (QFR) is an angiography-based method for the functional assessment of coronary arteries and Murray-law based QFR (µQFR) requires only a single coronary projection, enhancing its applicability. This study primarily aims to investigate the agreement between µQFR and 3D-QFR for functional assessment of donor vessels in CTO patients.</p> Methods <p>This single-center retrospective study enrolled patients with a single-vessel CTO lesion between January 2015 and June 2023. Both µQFR and 3D-QFR were used to assess the function of the predominant collateral donor artery following standard protocol.</p> Results <p>A total of 336 CTO patients were included. There was a good correlation between µQFR and 3D-QFR (<i>r</i> = 0.947, <i>p</i> &lt; 0.001). The diagnostic accuracy was 96.1% (95% CI, 93.5%–97.9%). ROC curve analysis revealed an AUC of 0.984 (95% CI, 0.971–0.997) for µQFR in estimating 3D-QFR.</p> Conclusions <p>µQFR has good correlation and agreement with standard 3D-QFR for the functional assessment of donor vessels in CTO patients.</p>