Precordial S-T segment mapping has been used to evaluate the extent of ischemie injury in patients with acute myocardial infarction. Because precordial S-T segment mapping is time-consuming and is limited to patients with anterior wall myocardial infarction, we evaluated the possibility of using the magnitude (ST-VM) and direction (ST-VD) of the S-T vector, derived from X, Y and Z leads of the Frank vector system, as a substitute for the precordial S-T segment mapping technique. Precordial S-T segment mapping and Frank system vectorcardiograms were simultaneously obtained in three groups: (1) nine normal subjects; (2) nine patients with persistent S-T segment elevation 2 to 15 months after acute anterior myocardial infarction; and (3) nine patients with acute anterior myocardial infarction studied on 41 occasions. For both systems the S-T segments were analyzed 20 and 60 msec after completion of inscription of the QRS complex. The sum of the S-T segment elevations for the 35 sites (∑ST) and the number of sites (NST) in which S-T segment elevations exceeded 0.1 mv were computed for the precordial S-T maps. The ST-VM and ST-VD were calculated by standard formulas from X, Y and Z lead tracings of the Frank vector system. Good correlations were observed between: ST-VM and ∑ST (r = + 0.818 and + 0.791 at 20 and 60 msec, respectively, P < 0.001); and ST-VM and NST (r = + 0.773 and + 0.705 at 20 and 60 msec, respectively, P < 0.001). Furthermore, changes in the location of S-T segment elevations in serial precordial S-T segment maps were reflected by changes in ST-VD. Observations in patients with inferior wall myocardial infarction suggest that ST-VM and ST-VD can be serially followed in such patients. Thus, estimation of the magnitude and direction of the S-T vector is a simple alternative to standard precordial S-T segment mapping that allows for continuous monitoring of S-T segment elevations in all patients with acute myocardial infarction.