Studies of patients during variant angina have revealed that there are specific changes in the terminal part of the QRS complex that provide information regarding the location of the ischemia. Extending these studies to acute myocardial infarction, the electrocardiogram (ECG) obtained from 32 patients within 5 h of the onset of chest pain was analyzed to determine if similar inferences could be made. A preinfarction ECG was available from each patient for comparison and 30 patients underwent coronary arteriography within 3 weeks of the infarction. The 10 patients with anterior infarction had a decrease (p less than 0.05) in the S wave in leads V2 (0.80 +/- 0.50 mV) and V3 (0.65 +/- 0.43 mV). In 23 patients with inferior infarction an increase (p less than 0.05) in the R wave of lead III (0.47 +/- 0.35 mV), S wave of lead aVL (0.31 +/- 0.23 mV) and R wave of lead aVF (0.37 +/- 0.30 mV) occurred. A strong positive correlation between the R wave changes in leads III and aVF and the S wave in lead aVL (r = 0.94 and 0.91, respectively) suggests that the R and S wave changes in these leads are expressions of the same phenomenon and indicates that the terminal QRS complex is chiefly affected. Eight of 23 patients with inferior infarction and ST depression in the anterior precordial leads had a normal left anterior descending coronary artery. All had an increase in S wave amplitude in leads V2 and V3. Eight patients had inferior infarction, ST depression in anterior leads and severe lesions in the left anterior descending artery or anterior wall motion abnormalities.(ABSTRACT TRUNCATED AT 250 WORDS)