Objective: To investigate the incidence and prognostic significance of fasting hyperglycemia in a large group of patients with a first myocardial infarction. Research Design and Methods: Blood glucose was measured after an 8-h overnight fast in 752 patients with a first myocardial infarction. Three groups of patients were identified: patients with normal fasting blood glucose (92.5%), patients with fasting hyperglycemia but no prior history of glucose intolerance (3%), and patients previously known to have diabetes mellitus (4.5%). Results: The fasting hyperglycemic patients were significantly older and had significantly more in-hospital complications than the normal blood glucose group. Previously known diabetic subjects tended to be older and had more mechanical complications postinfarction than the group with normal blood glucose but the difference did not reach statistical significance. There was no significant difference between the diabetic subjects and patients with fasting hyperglycemia in mean age and in-hospital prognosis. Conclusions: Fasting hyperglycemia detected after a first myocardial infarction is associated with a poor in-hospital prognosis that was not due to larger infarct size, as reflected in peak levels of cardiac enzymes. The measurement of a fasting blood glucose level provides additional information in identifying high-risk groups of patients postinfarction.