AIMS : Little is known about cardiac structure and function among atrial fibrillation (AF) subtypes; paroxysmal AF vs. persistent AF (PxAF), and across AF burden. We sought to assess differences in left atrial (LA) measures by AF subtype and burden. METHODS AND RESULTS : This was a cross-sectional echocardiographic substudy of a randomized trial of AF patients scheduled for catheter ablation. Patients had an echocardiogram performed 0-90 days prior to study inclusion. We performed conventional echocardiographic measures, left ventricular (LV) and LA speckle tracking. Measures were compared between AF subtype and burden (0%, 0-99%, and 99-100%) determined by 72-h Holter monitoring. Of 212 patients, 107 had paroxysmal AF and 105 had PxAF. Those with PxAF had significantly reduced systolic function (LV ejection fraction: 48% vs. 53%; P < 0.001), larger end-systolic and end-diastolic LA volumes (LAVi and LAEDVi), reduced LA emptying fraction (LAEF: 29% vs. 36%, P < 0.001), and reduced LA strain (LAs) (LAs: 20% vs. 26%, P < 0.001). LA measures remained significantly lower in PxAF after multivariable adjustments. All LA measures and measures of systolic function were significantly impaired in patients with 99-100% AF burden, whereas all measures were similar between the other groups (LAVi: 40mL/m2 vs. 33mL/m2 vs. 34mL/m2; LAEDVi: 31mL/m2 vs. 21mL/m2 vs. 22mL/m2, LA emptying fraction: 23% vs. 35% vs. 36%, LAs: 16% vs. 25% vs. 25%, for 99-100%, 0-99%, and 0% AF, respectively, P < 0.001 for all). These differences were consistent after multivariable adjustments. CONCLUSION : LA mechanics differ between AF subtype and burden and these characteristics influence the clinical interpretation of these measures.