Total body surface maps obtained from 19 patients with previous inferior myocardial infarction (IMI) were compared with maps obtained from 19 patients with left anterior fascicular block (LAFB) and six more patients in whom electrocardiographic changes were indistinguishable between IMI and LAFB. Three distinguishing features were detected: 1) abnormal high anterior positivity developed both in IMI and LAFB, but its onset was earlier in LAFB; 2) a broad rim of abnormal right lower negativity was seen in both groups, but in IMI it was within the first 40 msec, whereas in LAFB it was found in the middle and later parts of depolarization; 3) abnormal left lower negativity was seen in all the patients with LAFB, but was absent in IMI. Thus, despite similarities in the abnormalities detected, we found definite temporal and topographical differences that should aid in differentiating between IMI and LAFB in ambiguous cases.