Vectorcardiographic and pathologic studies were made on 21 cases with electrocardiographic incomplete left bundle branch block. Marked left ventricular hypertrophy, severe coronary sclerosis and myocardial infarction were more frequently observed in the presence of incomplete left bundle branch block than in the control group. A figure-of-eight pattern of the QRS loop in the horizontal plane, a classical pattern of incomplete left bundle branch block hitherto described, was observed only in 9 of 21 cases, although the normal initial deflection of the QRS loop directed to the right and anteriorly was absent in all the cases. The configuration of the QRS loop in the horizontal plane was very helpful for the diagnosis of associated myocardial infarction and marked left ventricular hypertrophy. A smooth figure-of-eight QRS loop in the horizontal plane was observed in the cases with marked left ventricular hypertrophy, while a bizarre distorted QRS loop with a figure-of-eight pattern was observed in the cases with extensive myocardial infarction. A T loop directed to the right and posteriorly was also considered to be a suggestive sign of myocardial infarction. The present observations clearly indicate the usefulness of vectorcardiograms for clinical diagnosis in the presence of electrocardiographic incomplete left bundle branch block. The results also suggest that it seems to be unlikely that one common mechanism is responsible for the development of electrocardiographic encomplete left bundle branch block.