This study aimed to determine the impact of dysglycemia on myocardial injury and cardiac dysfunction in acute myocardial infarctions (AMIs). From 2005 to 2016, a total of 1,593 patients with AMIs who underwent percutaneous coronary intervention were enrolled. The patients were classified into five groups according to the admission glucose level: ≤80, 81 to 140, 141 to 200, 201 to 260, and ≥261 mg/dL. The clinical and echocardiographic parameters and 30-day mortality were analyzed. The peak troponin I and white blood cell levels had a positive linear relationship to the admission glucose level. The left ventricular ejection fraction had an inverted U-shape trend, and the E/E' ratio was U-shaped based on euglycemia. The 30-day mortality also increased as the admission glucose increased, and the cut-off value for predicting the mortality was 202.5 mg/dL. Dysglycemia, especially hyperglycemia, appears to be associated with myocardial injury and could be another adjunctive parameter for predicting mortality in patients with AMIs.