Introduction: Heart failure and mildly reduced ejection fraction (HFmrEF) is associated with a favourable prognosis compared heart failure with reduced EF. However, HFmrEF may be a transitory state and serial imaging might clarify if these patients demonstrate an increasing, decreasing or stable EF. Hypothesis: LVEF trajectory can identify sub-groups of patients with HFmrEF with different prognosis. Materials and Methods: Patients with a diagnosis of HF and at least two echocardiograms performed ≥6 months apart were included if the LVEF measured 40-49% on the second study. They were classified based on change from their first echocardiogram as: a) HFmrEF-Incr if LVEF had increased ≥ 10% (n=450), b) HFmrEF-Dec if LVEF had decreased ≥10% (n=512), or c) HFmrEF-stable if they did not meet the other criteria (n=389). The primary outcome was a composite of all-cause mortality or cardiovascular hospitalization (ACM/CVH). Associations with time to first event were assessed with multivariable Cox analyses adjusted for age, medical history, and medications. Results: In total, 1351 patients with HFmrEF were included (median age 74, 35.8% women). During a median follow-up of 15.3 months, ACM/CVH occurred in 811 patients (324 ACM, 487 CVH). HFmrEF-Incr was associated with a lower incidence of ACM/CVH compared to patients with HFmrEF-Stable (adjusted HR 0.72, 95% CI 0.59 - 0.87, p Conclusions: Patients with HFmrEF and positive trajectory of LVEF were less likely to experience adverse outcomes, while those with a negative LVEF trajectory demonstrated a trend to higher risk. Categories based on LVEF trajectory provide clinically meaningful information and may help physicians make decisions regarding the need to pursue more aggressive medical therapy and surveillance.