Background: Patients with atrial fibrillation (AF) have an increased risk of diastolic dysfunction and heart failure. The purpose of this study was to identify independent predictors. of early (ie, only exercise induced) heart failure with preserved ejection fraction (HFpEF) and to describe the prevalence of early HFpEF among patients with paroxysmal AR. Methods and Results: One hundred patients with paroxysmal AF and preserved left ventricular ejection fraction (LVEF) underwent catheterization for left atrial pressure (LAP) measurements at rest and at the peak of arm exercise (LAP-exe). Based on resting and exercise LAP values, the patients were divided into 3 groups. Sixty-one patients had no evidence of HFpEF (LAP at rest <= 15 mm Hg, LAP-exe <25 mm Hg). Twenty-five subjects had early HFpEF (LAP at rest mm Hg, LAP-exe mm Hg, prevalence 25%). Fourteen patients already had HFpEF at rest (LAP at rest >15 mm Hg). Multivariate exact logistic regression analysis identified age years, LAP at rest mm Hg, and peak systolic mitral annular velocity <= 9.3 cm/s to be independent predictors of early HFpEF. Conclusions: In patients with paroxysmal AF and preserved LVEF, there appears to be a clinically significant prevalence of early HFpEF.