Body surface isopotential maps were recorded in 30 normal subjects and 32 patients with clinical evidence of old anterior myocardial infarction but without ECG findings of infarction. Position of the minimum in maps was compared at 5 msec intervals from the onset of QRS between the normal and infarction groups. A significant difference in the mean position of the minimum was observed at 5, 10, 15, 20, and 25 msec (p less than 0.001). The minima for the two groups achieved their clearest separation at 15 msec. Twenty-six of the 32 infarction patients showed the minima on the anterior chest, but 29 of the 30 normal subjects showed the minima on the back. These findings suggest the highly specific usefulness of body surface maps for the diagnosis of the presence of old anterior infarction undetectable by 12-lead ECGs.