Objective Some patients with heart failure with preserved ejection fraction (HFpEF) experience declining of left-ventricular ejection fraction (LVEF) during follow-up. We aim to investigate the characteristics and outcomes of patients with HF with declining ejection fraction (HFdEF). Methods We analyzed a prospective, nationwide multicenter cohort with consecutive patients with acute HF enrolled from March 2011 to December 2014. HFpEF was defined as LVEF >= 50% at index admission. After 1 year, HFpEF patients were further classified as HFdEF (LVEF >= 50% at admission and < 50% at 1 year), and persistent HFpEF (LVEF >= 50% both at admission and 1 year). Primary outcome was 4-year all-cause mortality according to HF type from HFdEF diagnosis. Results Of patients with HFpEF, 426 (90.4%) were diagnosed as having persistent HFpEF and 45 (9.6%) as having HFdEF. Natriuretic peptide level was an independent predictor of HFdEF (natriuretic peptide level > median: odds ratio: 3.20, 95% confidence interval [CI]: 1.42-7.25, P = 0.005). During 4-year follow-up, patients with HFdEF had higher mortality than those with persistent HFpEF (Log-rank P < 0.001). After adjustment, HFdEF was associated with an almost twofold increased risk for mortality (hazard ratio 1.82, 95% CI 1.13-2.96, P = 0.015). The use of beta-blockers, renin-angiotensin system inhibitors, and mineralocorticoid receptor antagonists was not associated with improved prognosis of patients with HFdEF. Conclusions HFdEF is a distinct HF type with grave outcomes. Further investigations that focus on HFdEF are warranted to better understand and develop treatment strategies for these high-risk patients. Graphic abstract [GRAPHICS] .