Serial precordial ST-segment ECG mapping with a grid consisting of 49 recording marks made on the anterior thorax of patients with acute anterior transmural myocardial infarction has been applied in the study of usefulness of this technique. It has been found that a pattern of variable devolution of the magnitude of ST-segment elevations is seen in uncomplicated myocardial infarction. Extension of the infarct has been characterized by re-elevation of ST-segments. Beneficial therapeutic interventions have resulted in reduction of the magnitude of ST-segment elevation. However, the technique cannot be applied in patients with inferior transmural myocardial infarction or in patients with functioning pacemakers, bundle branch blocks, or pericarditis. The significance of adherence to strict guidelines in performing ST-segment mapping and the analysis of mapping data in the light of the total clinical picture at the time of recordings is emphasized.