Background Increased left atrial (LA) volume after acute myocardial infarction (AMI) is an independent predictor of mortality and cardiovascular morbidity. However, it has not been established what are determinants of increment in LA volume. Methods and Results Forty patients with AMI who underwent successful PCI and cardiac MRI after PCI at baseline and 6 months were consecutively enrolled. Infarct size was assessed by the volume of area with delayed enhancement on MRI and expressed as percentage of LV mass. LA maximal volume index (LAVI_max) was calculated as LA volume at LV end systole (area-length method)/body surface area (ml/m²). Thirty three patients (82.5%) were acute STEMI and seven were acute NSTEMI (17.5%). LAVI_max significantly increased throughout follow-up (42.0 vs 48.4 ml/m², baseline vs 6 months, respectively, p=0.04). Baseline infarct size did not show relation to baseline LAVI_max. However, baseline infarct size was a determinant of LV end diastolic volume (LVEDV) at 6 months (r=0.57, p=0.001), LV end systolic volume (LVESV) at 6 months (r=0.62, p <0.001), and LAVI_max at 6 months (r=0.49, p=0.007). In multiple linear regressions, baseline infarct size showed positive relation to LAVI_max at 6 months independently of age, gender, and presence of hypertension, diabetes, mitral regurgitation, and atrial fibrillation. In addition, LVEDV and LVESV at 6 month showed positive relation to LAVI_max at 6 month (r=0.61, p=0.001; r=0.53, p=0.006, respectively). Conclusion In patients with AMI who underwent successful PCI, baseline infarct size was associated with LA remodeling in relation to LV remodeling.