This study described the linear relationship of the myocardial infarct size, measured postmortem, to the integral of the sequential vector spatial magnitudes analyzed at 400 samples per second during the initial period of abnormal depolarization (IAD) in 25 patients with myocardial infarction (MI) and ten controls. The equation for the fitted regression line was: %MI = -0.35 + 3.33 IAD; r = .90; p less than .001. The duration of abnormal depolarization was determined by two computer algorithms: first, using the classical method of pathological Q wave duration, whenever possible; and a new method of measurement of an abnormally slow rate of rise of vector spatial magnitude with time (dm/dt), when there were no Q waves in the Frank orthogonal lead input signals. There was not a significant difference in the mean values of IAD of the MI group or the control group calculated by the two algorithms.