OBJECTIVE Among patients with acute myocardial infarction (AMI) without known diabetes, hyperglycemia at admission is common and associated with worse outcomes. It may represent developing diabetes, but this association is unclear. Therefore, we examined the association between hyperglycemia (≥140 mg/dL) at admission and evidence of diabetes among patients with AMI without known diabetes within 6 months of their hospitalization. RESEARCH DESIGN AND METHODS We studied a national cohort of consecutive patients with AMI without known diabetes presenting at 127 Veterans Affairs hospitals between October 2005 and March 2011. Evidence of diabetes either at discharge or in the following 6 months was ascertained using diagnostic codes, medication prescriptions, and/or elevated hemoglobin A1c. Association between hyperglycemia at admission and evidence of diabetes was evaluated using regression modeling. RESULTS Among 10,499 patients with AMI without known diabetes, 98% were men and 1,761 (16.8%) had hyperglycemia at admission. Within 6 months following their index hospitalization, 208 patients (11.8%) with hyperglycemia at admission had evidence of diabetes comparedwith 443 patients (5.1%) without hyperglycemia at admission (P < 0.001). After multivariable adjustment, hyperglycemia at admission was significantly associated with subsequent diabetes odds ratio 2.56 (95% CI 2.15-3.06). Among those with new evidence of diabetes, 41%patients (267 of 651) had a hemoglobin A1c ≥6.5% without accompanying diagnostic codes or medication prescriptions, suggesting they had unrecognized diabetes. CONCLUSIONS Hyperglycemia at admission occurred in one of six patients with AMI without known diabetes and was significantly associated with new evidence of diabetes in the 6 months following hospitalization. In addition, two of five patients with evidence of diabetes were potentially unrecognized. Accordingly, diabetesscreening programs for hyperglycemic patients with AMI may be an important component of optimal care.