To assess the significance of ST segment shift during the acute phase of non-Q myocardial infarction we studied the clinic echocardiographic, ergometric and coronarographic findings of 46 patients with a first non-Q wave myocardial infarction. The study population was subdivided in 2 subgroups on the basis of acute electrocardiographic change (Group I with ST elevation, Group II with ST depression). Patients with ST elevation had little myocardial infarction with enzymatic (early CPK peak) and coronarographic (low prevalence of coronary occlusion) signs of early spontaneous fibrinolysis. The second group had more diffuse myocardial infraction, higher prevalence of multivessel coronary disease and positive stress test. The ECG changes in this subgroup an probably due to subendocardial necrosis for the presence of collateral flow. The worse intrahospital prognosis of patients with ST segment depression may be related to cardiac function and age.