Aims: To determine the prognostic impact of left ventricular ejection fraction (LVEF) trajectories from acute to chronic phase of acute myocardial infarction (AMI). Materials and methods: Among 2,266 consecutive patients admitted for AMI from February 2008 to January 2016, 1,330 survivors with LV ejection fraction (LVEF) ≥40% during hospitalization who had LVEF data at 6 months after AMI were analyzed. Patients were divided into three subgroups based on LVEF at 6 months: reduced-LVEF (<40%), mildly-reduced-LVEF (≥40% and <50%) and preserved-LVEF (≥50%). Occurrence of a composite of hospitalization for heart failure or cardiovascular death after 6 months of AMI was the primary endpoint. The prognostic impact of LVEF at 6 months on primary endpoint was analyzed with a multivariate-adjusted Cox proportional hazards regression model. Results: Overall, the mean age was 67.5 ± 11.9 years, and LVEF during index hospitalization was 59.4 ± 9.1%. The individual trajectories of LVEF from index hospitalization to 6 months was dynamically changed. The median (interquartile range) duration of follow-up was 3.0 (1.5-4.8) years, and the primary endpoint occurred in 35/1,330 (2.6%) patients (13/69 [18.8%] in the reduced-LVEF, 9/265 [3.4%] in the mildly-reduced-LVEF, and 13/996 [1.3%] in the preserved-LVEF categories). The adjusted hazard ratio for the primary endpoint in the reduced-LVEF vs. mildly-reduced-LVEF categories and in the reduced-LVEF vs. preserved-LVEF categories was 4.71 (95% confidence interval [CI], 1.83 to 12.13; p <0.001) and 14.37 (95% CI, 5.38 to 38.36; p <0.001), respectively (Figure). Conclusion: Late-onset LV systolic dysfunction was significantly associated with long-term outcomes after AMI. Our findings suggest that, even in AMI survivors without LV systolic dysfunction at the acute phase of AMI, reassessment of LVEF and careful monitoring of its trajectories is useful to predict long-term clinical outcomes.