Serial precordial mapping was done in 30 patients with acute anterior myocardial infarction, 27 transmural and 3 subendorcardial. The sum of ST elevations in the 48 lead map was designated as sigma ST. Normal sigma ST was calculated as 27.1+/-3.1 in males and 14.3+/-2.8 in females. In infarction it was 100.4+/-58.2 in males and 84.8+/-50.9 in females on the 1st day of admission. Sigma ST was elevated on day 1 and tended to fall gradually. In 6 patients it was normal by day 7 and in 7 it was still abnormal by day 21. This fall roughly correlated with fall in SGOT and CPK levels. Significant elevation of sigma ST occurred in 14 of 27 cases after day 1. In 10 of 27 cases significant re-elevation occurred on day 4 or after i.e. when the patient was outside the ICCU. In 10 of the 14 re-elevations there was pain or worsening of clinical picture and in 12 there was re-elevation of SGOT. This elevation presumably implied infarct extension. There was a tendency to more arrhythmias in the patients with higher sigma ST and of the 4 deaths in the series 3 had very high sigma ST and high levels of SGOT. The patient with the highest sigma ST 295 died in cardiogenic shock. The number of risk factors was found to be higher in the high sigma ST group.