The two current criteria for diagnosis of left anterior fascicular block (LAFB) were evaluated; they are marked left axis deviation (LAD) and a delay in the time of inscription of the intrinsicoid deflection (ID) in lead aVL asynchronous to V6. From 400 electrocardiograms with a LAD of --30 degrees or greater, 62 percent showed asynchronous activation of the left ventricle. There was only a general relationship between the degree of LAD and delayed ID in aVL. The incidence of delayed ID in aVL was as follows: 2 percent with mean frontal QRS axis at 0 degrees; 9 percent at --15 degrees; 41 percent at --30 degrees; 69 percent at --45 degrees; 82 percent at --60 degrees; and 100 percent at --75 degrees or greater. The lack of correlation between both criteria in many instances questions their validity. The LAD alone should not be considered synonymous with LAFB. Recognition of delayed inscription of the ID in aVL is a useful supplemental criterion for diagnosis.