AIMS: To assess the relationship between left atrial (LA) size and outcome after high-risk myocardial infarction (MI) and to study dynamic changes in LA size during long-term follow-up. METHODS AND RESULTS: The VALIANT Echocardiography study prospectively enrolled 610 patients with left ventricular (LV) dysfunction, heart failure (HF), or both following MI. We assessed LA volume indexed to body surface area (LAVi) at baseline, 1 month, and 20 months after MI. Baseline LAVi was an independent predictor of all-cause death or HF hospitalization (P = 0.004). In patients who survived to 20 months, LAVi increased a mean of 3.00 +/- 7.08 mL/m(2) from baseline. Hypertension, lower estimated glomerular filtration rate, and LV mass were the only baseline independent predictors of LA remodelling. Changes in LA size were related to worsening in MR and increasing in LV volumes. LA enlargement during the first month was significantly greater in patients who subsequently died or were hospitalized for HF than in patients without events. CONCLUSION: Baseline LA size is an independent predictor of death or HF hospitalization following high-risk MI. Moreover, LA remodelling during the first month after infarction is associated with adverse outcome.