Background: Determination of myocardial blood flow (MBF) with MRI is usually performed with dynamic contrast enhanced imaging (MBF_DCE). MBF can also be determined from coronary sinus blood flow (MBF_CS), which has the advantage of being a noncontrast technique. However, comparative studies of MBF_DCE and MBF_CS in large cohorts are lacking. Purpose: To compare MBF_CS and MBF_DCE in a large cohort. Study Type: Prospective, sequence-comparison study. Population: 147 patients with type 2 diabetes mellitus (age: 56+/−12 years; 106 male; diabetes duration: 12.9+/−8.1 years), and 25 age-matched controls. Field Strength/Sequences: 1.5 Tesla scanner. Saturation recovery sequence for MBF_DCE vs. phase-contrast gradient-echo pulse sequence (free-breathing) for MBF_CS. Assessment: MBF_DCE and MBF_CS were determined at rest and during coronary dilatation achieved by administration of adenosine at 140 μg/kg/min. Myocardial perfusion reserve (MPR) was calculated as the stress/rest ratio of MBF values. Coronary sinus flow was determined twice in the same imaging session for repeatability assessment. Statistical Tests: Agreement between MBF_DCE and MBF_CS was assessed with Bland and Altman's technique. Repeatability was determined from single-rater random intraclass and repeatability coefficients. Results: Rest and stress flows, including both MBF_DCE and MBF_CS values, ranged from 33 to 146 mL/min/100 g and 92 to 501 mL/min/100 g, respectively. Intraclass and repeatability coefficients for MBF_CS were 0.95 (CI 0.90; 0.95) and 5 mL/min/100 g. In Bland–Altman analysis, mean bias at rest was –1.1 mL/min/100 g (CI −3.1; 0.9) with limits of agreement of −27 and 24.8 mL/min/100 g. Mean bias at stress was 6.3 mL/min/100 g (CI −1.1; 14.1) with limits of agreement of −86.9 and 99.9. Mean bias of MPR was 0.11 (CI: −0.02; 0.23) with limits of agreement of −1.43 and 1.64. Conclusion: MBF may be determined from coronary sinus blood flow, with acceptable bias, but relatively large limits of agreement, against the reference of MBF_DCE. Level of Evidence: 1. Technical Efficacy Stage: 2.