A total of 21 patients, 10 with anterior and 11 with inferior myocardial infarction (MI) had serial precordial 49-lead ECG maps or standard ECGs on admission, and 2, 6, 12, 24 hours, and 7 days thereafter, to evaluate the natural course of ST-segment elevation (index of ischemic injury) and QRS changes (index of necrosis), and their relationship. ECG parameters used included the sum of ST-segment elevations, number of sites showing such changes, the sum of R waves from all leads with ST-segment elevations, and a QRS score of all leads showing ischemic injury on admission. Sums of ST-segment elevations either did not show statistically significant change throughout the study or showed unexplained re-elevation (anterior MI), or completed their downward course in 6 hours (inferior MI). The number of sites showing ST-segment elevations either remained unchanged (inferior MI), or declined at 2 hours without further change. However changes in the sum of R waves or QRS scores were gradual and completed most of their overall course within 24 hours. Decline of sums of R waves correlated well with sums of ST-segment elevations on admission (inferior MI), and the number of sites showing changes (anterior MI). The course of changes in QRS complexes is more reliable than the alterations of ST-segment elevations for the monitoring of evolution of MI. The former, assessed frequently in the first 24 hours, should be useful in the evaluation of therapies for patients with MI.