Objective This observational study aimed to examine the prognostic association of angiotensin-converting enzyme inhibitors (ACE-I)/angiotensin receptor blockers (ARB) in different left ventricular ejection fraction (LVEF) categories. Methods In 3717 patients enrolled in the KCHF Registry, a multicentre registry including consecutive patients hospitalized for acute heart failure (HF), we assessed patient characteristics and association between ACE-I/ARB and clinical outcomes according to LVEF. In the three LVEF categories (reduced LVEF HFrEF, mid-range LVEF HFmrEF and preserved LVEF HFpEF), we compared the patients with ACE-I/ARB as discharge medication and those without, and assessed their 1-year clinical outcomes. We defined the primary outcome measure as a composite of all-cause death and HF hospitalization. Results The 1-year cumulative incidences of the primary outcome measure were 36.3% in HFrEF, 30.1% in HFmrEF and 33.8% in HFpEF (log-rank P = 0.07). The adjusted risks of the ACE-I/ARB group relative to the no ACE-I/ARB group for the primary outcome measure were significantly lower in HFrEF and HFmrEF (HR 0.66 95%CI 0.54-0.79, P0.001, and HR 0.61 0.45-0.82, P = 0.001, respectively), but not in HFpEF (HR 0.95 0.80-1.14, P = 0.61). There was a significant interaction between the LVEF category and the ACE-I/ARB use on the primary outcome measure (P.sub.interaction = 0.01). Conclusions ACE-I/ARB for patients who were hospitalized for acute HF was associated with significantly lower risk for a composite of all-cause death and HF hospitalization in HFrEF and HFmrEF, but not in HFpEF. ACE-I/ARB might be a potential treatment option in HFmrEF as in HFrEF.