Acute myocardial infarction may be associated with the development of Q waves on the electrocardiogram (ECG), or with changes limited to the ST segment or T wave. The ECG changes do not accurately differentiate transmural from nontransmural infarction. However, the presence or absence of a Q wave does correlate with some aspects of the clinical course of patients after myocardial infarction, and is therefore of prognostic value. Q-wave infarctions are more likely to be complicated by congestive heart failure during hospitalization. The in-hospital mortality is also higher after a Q-wave infarction than after a non-Q infarction. Both of these findings are probably due to the association of a Q wave with a larger mass of infarcted myocardium. The long-term mortality, however, is the same for Q-wave and non-Q-wave infarctions. This is probably due to an increased late mortality after non-Q infarctions, related in part to a higher rate of reinfarction. The differences between Q-wave and non-Q-wave infarctions are not due to obvious differences in extent and location of coronary artery obstructions. However, there may be differences in the collateral circulation, with more extensive collaterals associated with non-Q infarcts. Appreciation of the prognostic significance of the ECG changes in acute myocardial infarction may help direct the evaluation and management of the patient after myocardial infarction.