To assess whether gross pathologic differences exist between hearts with left bundle branch block (LBBB) and left-axis deviation (LAXD) and those with LBBB and a normal frontal plane axis, we examined 70 hearts with LBBB in a series of 1410 sequential dissections (5%). Thirty-two hearts had LAXD and 34 had normal axes on the correlative ECG. Left ventricular enlargement occurred frequently (93%). No significant differences were found in age distribution, left ventricular weight, coronary anatomy or infarct location. Quantitative analysis revealed larger inferoposterolateral and apical infarcts in hearts with LBBB and LAXD (p less than 0.01). The accuracy of various electrocardiographic signs of left ventricular enlargement and myocardial infarction in the presence of LBBB was assessed. Voltage criteria and QRS duration poorly define anatomic chamber enlargement. Anterior infarction is suggested by a q or pathological Q wave in lead I, a q wave in leads I, V5 and V6, or notched S waves in V3 or V4. Pathologic q waves or ST shifts in the inferior leads have high diagnostic specificity but low sensitivity for inferior infarction.