The degree of vectorcardiographic ST segment elevation was employed as an index of myocardial ischemic injury in a study of 27 patients after acute myocardial infarction (AMI). The ST segment vector magnitude (STVM) was derived from the continuously modified Frank vectorcardiogram and was plotted serially by hours after onset of AMI. The STVM in normal subjects was 51.1 ± 7.1 μV (mean ± SE). A standard deviation of the pooled variance of 15.2 μV was obtained in a group of control patients and a change of more than 2SD (>30 μV) in an individual STVM was considered to be significant. The STVM progressively decreased in patients who survived without clinical complications while it remained elevated in those with congestive heart failure. A modest, sustained re elevation of STVM was observed in patients who developed pericarditis, and a significant late average of 64 μV occurred in survivors with infarct extension. In contrast, STVM underwent a major increase in patients who died. In five of these six patients without associated pericarditis a mean increase of 164 μV was recorded in the last 5-12 hours of life. Whle death was clinically predictable in two patients with cardiogenic shock, it was not so for the four other patients who died. Thus, major increases in STVM frequently suggested significant new ischemic injury and were often premonitory to sudden death after AMI. The increases preceding death implied that not only ventricular ectopy but also lethal conduction abnormalities after AMI might be ischemia related.