Thirteen hearts from subjects (10 male, 3 female; mean age 65 years) with left posterior fascicular block were studied. Left posterior fascicular block was associated with right bundle branch block in nine cases and alternated with left anterior fascicular block in three. In nine of the patients, the conduction disorders were observed during an episode of acute myocardial infarction. Of these nine patients, four showed interruptive lesions at the level of the posterior radiation of the left bundle or of the posterior portion of the main left bundle branch. Two were found to have severe alterations scattered throughout the left bundle branch system: One of them had alternating left anterior and left posterior fascicular block; and the other manifested complete heart block in the course of her illness. No interruptive changes of the posterior fibers were found in three cases in which the heart was obtained early after death. All 9 patients had severe coronary artery disease (six had triple vessel disease and three of the six had a left main coronary arterial stenosis or obstruction; two patients had double vessel disease). Among the four patients with chronic left posterior block, three were found to have heavy calcifications of the left side of the cardiac skeleton; the remaining one had diphtheritic myocarditis. All had major alterations of the left-sided conduction system that were consistently maximal at the level of the posterior fibers or posterior portion of the main left bundle branch. In two of these patients scattered lesions were found throughout the left bundle branch fibers; one had alternating left anterior and left posterior fascicular block and the other had complete heart block. In general, the alterations underlying left posterior fascicular block were less widely spread than in left anterior fascicular block; however, they were more severe and more proximally located.