Heart failure with mildly-reduced ejection fraction (HFmrEF) of 40%-49% is an under-recognized type of heart failure. The percentage of HFmrEF of the HF population is between 13% and 24%, suggesting that approximately 1.6 million individuals in the United States have HFmrEF.1 The majority of landmark heart failure (HF) studies historically excluded patients with this borderline ejection fraction (EF). While main studies of heart failure with reduced ejection fraction (HFrEF) studied patients with EF of ≤35%-40%2, 3, 4, 5, 6, 7, 8, 9 with some studies that included only severely reduced EF of <25%,10 studies of heart failure with preserved ejection fraction (HFpEF) focused on patients with EF ≥45%.11, 12, 13 Patients with HFmrEF remain in the “gray” under-studied area in terms of their prognosis and their predictors of outcomes. It has been debated whether patients with HFmrEF clinically behave similarly to patients with HFrEF or patients with HFpEF in terms of their morbidity and mortality, represent a transitional phenotype between the 2, or have their distinct pattern of prognosis. On the other hand, many studies demonstrated that the change in EF was associated with a change in clinical phenotype and outcomes.14,15 To our knowledge, patients with stable mildly reduced EF over one year were not primarily studied before. The primary aims of our paper were to study the prognosis of patients with stable HFmrEF of one year (in comparison to patients with stable HFrEF and patients with stable HFpEF), whether having stable EF results in similar clinical outcomes in the three HF classes, and predictors of clinical outcomes and length of hospital stay of patients with stable HFmrEF.