BACKGROUND Acute glycometabolic derangement in non-diabetic patients with acute myocardial infarction (AMI) has been reported with discrepant prognostic results. The aim of the present study was to assess the prognostic impact of glycated haemoglobin (HbA1c) levels, reflecting long-term glycometabolic disturbance, in a population of patients without known diabetes mellitus. METHODS We examined 601 consecutive prospective patients diagnosed with AMI and unknown diabetes mellitus. We analysed metabolic function as a stratified variable using three groups of patients according to HbA1c: Group 1 (< 5.5%): 222 patients (37%); Group 2 (5.5 to 6.4%): 337 patients (56%); Group 3 (>6.4%): 42 patients (7%). Association between HbA1c groups and classic cardiovascular risk factor and in-hospital outcomes were assessed through univariate and multivariate analysis. RESULTS In-hospital mortality was 5% (32/601 patients). Higher HbA1c was associated with poor glycometabolic control, older patients, obesity, hypertension, Killip's class>1, increased heart rate, initial bundle branch block, atrial fibrillation and higher mortality during follow-up. In a multivariate adjusted risk, in-hospital mortality was associated with age (odds ratio (OR)= 1.056; 1-1.1; p=0.006), Killip's class>1 (OR=2.4; 1-6.1; p=0.05) and HbA1c (OR=1.5; 1.15-1.9; p=0.002). Hypertension (OR=0.39; 0.18-0.87; p=0.022) and angiotensin-converting enzyme inhibitors (OR=0.28; 0.12-0.69; p=0.005) were protective factors. CONCLUSIONS HbA1c is an important risk marker in the absence of a history of diabetes mellitus in patients with AMI. The optimal management strategy in these patients may contribute to decreased in-hospital mortality.