A study correlating the electrocardiographic findings and the histology of the atrioventricular (AV) conduction system was carried out in 14 cases with chronic complete AV block and in 13 cases without chronic complete AV block. Patients with chronic complete AV block were divided into two groups, based on the width of the QRS complex. The QRS complexes were narrow (less than 0.12 second) in four cases (group 1) and wide (greater than or equal to 0.12 second) in 10 cases (group 2). In group 1, the main lesion was located in the penetrating portion of His bundle (Hisp) in one heart, in the branching portion of the His bundle (Hisb) in another and in the combined regions of Hisb and the left bundle branch in two. Three of the four cases in group 1 had idiopathic fibrosis of the conduction system and one had calcific nodules in the central fibrous body. In group 2, the main lesion was located in Hisb in two cases, in the combined regions of Hisb and the right bundle branch in one, in the Hisb and in the bilateral bundle branches in two, and in the bilateral bundle branches in five. All cases in group 2 were of the idiopathic type, except case 5, which had calcific aortic stenosis. In 13 cases without chronic complete AV block the AV conduction system was histologically normal, except for slight-to-moderate aging changes in the His bundle or the bundle branches. Lesions of the Hisb, which is believed to be the "distal His" electrophysiologically, may induce complete AV block with narrow or wide QRS complexes, depending upon the severity of the lesions in Hisb or adjacent bundle branches.