AIMS: Tailored heart failure treatment and risk assessment in patients following ST-segment elevation myocardial infarction (STEMI) is mainly based on the assessment of the left ventricular (LV) ejection fraction (EF). Assessment of the final infarct size in addition to the LVEF may improve the prognostic evaluation. To evaluate the prognostic importance of the final infarct size measured by cardiovascular magnetic resonance (CMR) in patients with STEMI. METHODS AND RESULTS: In an observational study the final infarct size was measured by late gadolinium enhancement CMR 3 months after initial admission in 309 patients with STEMI. The clinical endpoint was a composite of all-cause mortality and admission for heart failure. During the follow-up period of median 807 days (IQR: 669-1117) 35 events (5 non-cardiac deaths, 3 cardiac deaths, and 27 admissions for heart failure) were recorded. Patients with a final infarct size ≥median had significantly higher event rates than patients with a final infarct size