This study concerns two cases of chronic 2 degrees atrioventricular (A-V) block with left bundle branch block (LBBB). Pathological studies included serial section of the conduction systems. Case 1 had type I 2 degrees block with LBBB. Electrophysiological studies revealed type I 2 degrees block proximal to the His bundle recording site and a prolonged H-V interval (60 msec). Pathologically there was a moderate to marked fibrosis of the approaches to the A-V node and of the A-V node, marked fibrosis of the left bundle branch, and moderate involvement of the right bundle branch. The changes proximal to the His bundle were more marked than the changes distal to this bundle. Case 2 had type II and 2:1 2 degrees A-V block with LBBB. Electrophysiologically the site of block was distal to the His bundle recording site, and there was a prolonged A-V node and severe involvement of both bundle branches. The changes distal to the His bundle were more severe than the changes proximal to the His bundle. This study reveals that the electrophysiologic data more closely approximated the pathologic findings than did surface electrocardiographic data alone. It also emphasizes that there may be multiple sites of disease in chronic 2 degrees block with bundle branch block.