Background Many hospitalized patients with heart failure and reduced ejection fraction ( HF r EF ) have a slow heart rate at discharge, and the effect of β-blockers may be reduced in those patients. We sought to examine the variable effect of β-blockers on clinical outcomes according to the discharge heart rate of hospitalized HF r EF patients. Methods and Results The KorAHF (Korean Acute Heart Failure) registry consecutively enrolled 5625 patients hospitalized for acute heart failure. In this analysis, we included patients with HF r EF (left ventricular ejection fraction ≤40%). Slow heart rate was defined as <70 beats per minute regardless of the use of β-blockers. The primary outcome was 1-year all-cause postdischarge death according to heart rate. Among 2932 patients with HF r EF , 840 (29%) had a slow heart rate and 56% received β-blockers at discharge. Patients with slow heart rates were older and had lower 1-year mortality than those with high heart rates ( P<0.001). A significant interaction between discharge heart rate and β-blocker use was observed ( P<0.001 for interaction). When stratified, only patients without a β-blocker prescription and with a high heart rate showed higher 1-year mortality. In a Cox-proportional hazards regression analysis, β-blocker prescription at discharge was associated with 24% reduced risk for 1-year mortality in patients with high heart rates (hazard ratio: 0.76; 95% CI, 0.61-0.95) but not in those with slow heart rates (hazard ratio: 1.02; 95% CI, 0.68-1.55). Conclusions Many patients with acute heart failure have slow discharge heart rates, and β-blockers may have a limited effect on HF r EF and slow discharge heart rate. Clinical Trial Registration URL : http://www.clinicaltrial.gov . Unique identifier: NCT 01389843.