The electrocardiographic criteria for isolated left anterior fascicular block are reviewed and illustrated. Left anterior fascicular block decreases the voltage in the chest leads and increases the voltage in the limb leads. The usual voltage criteria of left ventricular hypertrophy must be modified appropriately. Changes in repolarization include a decrease in the T wave of leads I and AVL and an increase in leads II, III, AVF, V5 and V6. Small Q waves in V2 may simulate an anteroseptal myocardial infarction. Three criteria for the possible diagnosis of superimposed inferior myocardial infarction have been suggested. Both inferior and anterior myocardial infarctions may be masked by R waves replacing Q waves. In the presence of a recent anterior infarction, right bundle branch block may also be masked. Thus, left anterior fascicular block may mask or mimic infarction and left ventricular hypertrophy and mask right bundle branch block in the setting of an acute anterior myocardial infarction.