To elucidate the mechanism, cardiac function and prognosis of complete left bundle branch block (CLBBB), 32 patients with CLBBB were studied using echocardiographic and vectorcardiographic methods. They were classified into 5 types: according to the presence or absence of early systolic notch and paradoxical motion of interventricular septum. Type I, an early systolic notch and paradoxical motion of the interventricular septum; type II, an early systolic notch and a flat septal motion, type III, an early systolic notch but no paradoxical motion; type IV, an early systolic notch of a slight degree but no paradoxical motion; type V, showed neither early systolic notch nor paradoxical motion. Type I patients had enlarged left ventricular dimension, severe cardiac damage and poor prognosis while type IV patients had a good ventricular function and prognosis with no complications. In type V patients, intraventricular conduction disturbances were apparent; a wide QRS loop in horizontal plane with no significant terminal delay. In type II and III, there were no typical findings. The histology revealed that the left bundle branch block was located at the bifurcation.