We have examined the relation between electrocardiographic ST elevation during treadmill exercise (greater than or equal to 1 mm, using the conventional 12 leads), the severity of coronary artery disease, and left ventricular wall motion abnormalities in 680 patients. They were divided into three groups: (1) 218 patients with clinically significant coronary artery disease, (2) 178 patients with clinically significant coronary artery disease, and (3) 284 patients with clinically significant coronary artery disease and previous myocardial infarction. ST elevation during exercise (predominantly in lead V2) was seen in two patients (1%) in group 1, three patients (2%) in group 2, and 147 patients (52%) in group 3. Coronary artery disease (number of vessels involved and severity of stenoses) was comparable in groups 2 and 3. All the patients in group 1 showed a normal left ventricular contraction pattern; 64% of the patients in group 2 showed wall motion abnormalities (predominantly hypokinesia) and 95% of group 3 (mainly akinesia, dyskinesia, or aneurysm). A strongly positive correlation was seen between the ST elevation and left ventricular dysfunction in patients belonging to group 3. The overall sensitivity and the specificity of the stress test in detecting wall motion abnormalities was 55% and 100% respectively. The sensitivity increased with deterioration in left ventricular function, reaching 81% and 90% in patients with dyskinesia and aneurysm, respectively. Maximal ST elevation (greater than or equal to 3 mm) was confined to the patients with dyskinesia or aneurysm. The incidence of ST elevation during exercise was also related to the location of previous infarction, showing a positive response in 85% of patients with anterior myocardial infarction and in only 33% with inferior myocardial infarction. We conclude that ST segment elevation during exercise in patients with previous myocardial infarction is a sensitive and a specific indicator of advanced left ventricular asynergy. The ST segment response during exercise in patients with previous infarction and with angiographically demonstrated myocardial asynergy appears to be a continuous spectrum. A normal ST segment response or elevation alone usually signifies involvement of only one vessel supplying the infarcted myocardium, ST elevation with concomitant ST depression indicates additional coronary artery disease, and ST depression alone indicates overwhelming myocardial ischaemia resulting from multiple vessel disease. The employment of multiple leads is essential to obtain this information.