Aims/Introduction: Admission hyperglycemia is associated with poor outcome in patients with myocardial infarction. The present study evaluated the relationship between admission glucose level and other clinical variables in patients with ST-elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PCI). Materials and Methods: The 959 consecutive STEMI patients undergoing primary PCI were divided into five groups based on admission glucose levels of <100, 100-139, 140-189, 190-249 and ≥250 mg/dL. Their short-and long-term outcomes were compared. Results: Higher admission glucose levels were associated with significantly higher inhospital morbidity and mortality, the overall mortality rate at follow up, and the incidence of reinfarction or heart failure requiring admission or leading to mortality at follow up. The odds ratios (95% confidence interval) for in-hospital morbidity, in-hospital mortality, mortality at follow up and re-infarction or heart failure or mortality at follow up of patients with admission glucose levels ≥190 mg/dL, compared with those with admission glucose levels <190 mg/dL, were 2.12 (1.3-3.4, P = 0.001), 2.74 (1.4-5.5, P = 0.004), 2.52 (1.2-5.1, P = 0.01) and 1.70 (1.03-2.8, P = 0.04), respectively. Previously non-diabetic patients with admission glucose levels ≥250 mg/dL had significantly higher in-hospital morbidity or mortality (44 vs 70%, P = 0.03). Known diabetic patients had higher rates of reinfarction, heart failure or mortality at follow up in the 100-139 mg/dL (8 vs 27%, P = 0.04) and 140-189 mg/dL (11 vs 26%, P = 0.02) groups. Conclusions: Admission hyperglycemia, especially at glucose levels ≥190 mg/dL, is a predictor of poor prognosis in STEMI patients undergoing primary PCI.