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Background: Atrial fibrillation (AF) is the most common performance limiting arrhythmia in racehorses. High dose exercise and airway disease promote AF in humans. Few studies have investigated epidemiological factors associated with AF in horses. Objectives: Explore variables relating to performance, exercise volume and postrace endoscopic findings in horses with AF. Animals: A total of 164 horses with poor race performance and postrace AF were compared to 321 horses with poor performance without AF (PP) and 314 horses performing to expectation (TE). Methods: Horse‐level and race‐level variables for horses racing in Australia and Hong Kong from 2009 to 2021 were compared using univariable and multivariable logistic regression. Postrace endoscopic exercise‐induced pulmonary hemorrhage (EIPH) and tracheal mucus accumulation (TMA) grades for AF and PP horses were compared using chi‐squared analysis. Results: Variables that were significant in the multivariable model of AF compared to TE were distance (lengths) behind the winner, (odds ratio [OR]; 95% confidence interval [95% CI], 1.41 [1.32‐1.51], P <.0001), cumulative prize money per start before the event (OR [95% CI] 1.02 [1.01‐1.03], P =.01) and age (OR [95% CI] 0.72 [0.55‐0.92], P =.01). More AF horses had EIPH grade ≥3 (23/109; 21.1%) than PP horses (7/213; 3.3%; OR [95%CI] 7.9 [3.3‐20.2], P <.0001). Conclusions and Clinical Importance: Acute race performance was substantially impaired by AF but career earnings before the event were not inferior. Exercise volume did not promote AF. Higher grades of EIPH found in AF horses suggests a mechanistic relationship between these conditions. [ABSTRACT FROM AUTHOR]