Objectives. The purpose of this study was to evaluate the long-term prognostic value of ST segment depression on the electrocardiogram (ECG) in patients with acute myocardial infarction. Background. The prognostic importance of ST depression on the ECG has been studied in small groups of patients with infarction, but larger numbers are needed. Methods. Coronary care unit ECGs of 1,234 patients who survived the coronary care unit with acute Q wave (n = 896) or non-Q wave (n = 338) myocardial infarction were analyzed for the presence of ST segment depression. Patients were followed up for up to 4 years. Results. ST segment depression was present in 607 patients. Those with ST segment depression had a 1-year mortality rate of 10.3% compared with a rate of 5.6% for those without ST segment depression (p = 0.002). This effect was seen in both the Q wave and non-Q wave subgroups, of the 437 patients with anterior ST segment elevation, those with ST segment depression in other regions had a 13,6% 1-year mortality rate compared with a rate of 6.9% for those with no ST segment depression (p = 0.0005). Of the 514 patients with inferior ST segment elevation, those with ST segment depression in other leads had an 11.0% 1-year mortality rate compared with a 1.8% rate for those with no ST segment depression (p = 0.0001). The Cox proportional hazards model showed that ST segment depression was an independent predictor of mortality over the follow-up period. Conclusions. ST segment depression on the admitting ECG in patients with acute myocardial infarction is a predictor of increased mortality in the year after infarction.