Left atrial myopathy has been implicated in atrial fibrillation (AF)-related stroke and embolic stroke of undetermined source (ESUS).To use advanced cardiac magnetic resonance (CMR) imaging techniques, including left atrial (LA) strain and 4D flow CMR, to identify atrial myopathy in patients with ESUS.20 patients with ESUS and no AF or other cause for stroke, and 20 age and sex-matched controls underwent CMR with 4D flow analysis. Markers of LA myopathy were assessed including LA size, volume, ejection fraction, and strain. 4D flow CMR was performed to measure novel markers of LA stasis such as LA velocities and the LA residence time distribution time constant (RTDtc). These markers of LA myopathy were compared between the two groups.There was no significant difference in: CMR-calculated LA velocities or LA total, passive or active ejection fractions between the groups. There was no significant difference in CMR-derived reservoir, conduit or contractile average longitudinal strain between the ESUS and control groups (22.9 vs 22.6%, p=0.379, 11.2 ± 3.5 vs 12.4 ± 2.6% p=0.224, 10.8 ± 3.2 vs 10.4 ± 2.3%, p=0.625 respectively). Similarly, RTDtc was not significantly longer in ESUS patients compared to controls (1.3 ± 0.2 vs 1.2 ± 0.2, p=0.1).There were no significant differences in any CMR marker of atrial myopathy in ESUS patients compared to healthy controls, likely reflecting the multiple possible aetiologies of ESUS suggesting that the role LA myopathy plays in ESUS is smaller than previously thought.