OBJECTIVES The study sought to assess the independent risk factors for, consequences of, and outcomes with atrial fibrillation (AF) compared with sinus rhythm (SR) in heart failure (HF) with preserved ejection fraction (HFpEF) versus HF with mid-range ejection fraction (HFmrEF) versus HF with reduced ejection fraction (HFrEF). BACKGROUND AF is common in HF, but most data are from HFrEF. The importance of AF in HFpEF and HFmrEF is less well known. METHODS In patients from 2000 to 2012 in the SwedeHF (Swedish Heart Failure Registry) registry, enriched with patient-level data from national health care registries, the authors assessed prevalence of, associations with, and prognostic impact of AF in HFpEF versus HFmrEF versus HFrEF. RESULTS Of 41,446 patients, 23% had HFpEF, 22% had HFmrEF, and 55% had HFrEF. The prevalence of AF was 65%, 60%, and 53% in HFpEF, HFmrEF, and HFrEF, respectively. Independent associations with AF were similar in HFpEF, HFmrEF, and HFrEF and included greater age, male, duration of HF, prior myocardial infarction, and prior stroke or transient ischemic attack (TIA). The adjusted hazard ratios for AF versus SR in HFpEF, HFmrEF, and HFrEF were the following: for death, 1.11 (95% confidence interval [CI]: 1.02 to 1.21), 1.22 (95% CI: 1.12 to 1.33), and 1.17 (95% CI: 1.11 to 1.23); for HF hospitalization or death, 1.17 (95% CI: 1.09 to 1.26), 1.29 (95% CI: 1.20 to 1.40), and 1.15 (95% CI: 1.10 to 1.20); and for stroke or TIA or death, 1.15 (95% CI: 1.07 to 1.25), 1.23 (95% CI: 1.13 to 1.34), and 1.19 (95% CI: 1.14 to 1.26). CONCLUSIONS AF was progressively more common with increasing ejection fraction, but was associated with similar clinical characteristics in HFpEF, HFmrEF, and HFrEF. AF was associated with similarly increased risk of death, HF hospitalization, and stroke or TIA in all ejection fraction groups. In contrast, AF and SR populations were considerably different regarding associated patient characteristics and outcomes. (C) 2017 by the American College of Cardiology Foundation.