The finding of ST segment depression (reduced ST) in the anterior leads during acute inferior myocardial infarction is a common clinical sign. Nevertheless, its significance is not yet well established. To evaluate the significance of this finding, 58 patients with acute inferior myocardial infarction, who has an electrocardiogram within 8 hours from the onset of the disease, were divided into 2 groups: group A (14 patients with anterior reduced ST less than 1 mV) and group B (44 patients with reduced ST greater than or equal to 1 mV in one or more anterior leads). All patients subsequently underwent coronary angiography and left ventriculography, mean 50 days after acute myocardial infarction. reduced ST was not predictive of left anterior descending coronary artery disease. On the contrary, a significantly higher rate of 2-3 vessel disease (p less than 0.05) and of critical stenosis or occlusion of the right or circumflex coronary artery (p less than 0.05) was found in group B. Peak CK level was significantly higher (p less than 0.01) in this group as well. No significant difference was found in ejection fraction and anterior wall motion abnormalities, whereas a higher number of patients in group B showed a depressed function of the postero-basal segment (p less than 0.05). During 6 months follow-up, 2 patients in group A and 24 in group B experienced cardiac events (angina, reinfarction, heart failure, coronary artery by-pass grafting, cardiac death) (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)