Fifty-six patients with acute transmural anterior wall myocardial infarction (MI) were investigated with a 24-electrode grid and 34 patients with an acute transmural inferior wall MI were investigated with standard ECG leads II, III, and a VF in order to study the length of time after the onset of pain during which the development of Q waves and reduction of R waves progress. These ECG changes continued for 18-26 h after onset of pain but the majority appeared during the first 12 h.