Abnormalities of cardiac conduction have become the subject of intensive clinical investigation as more is learned of their natural history and as therapy becomes more rational and effective. In this report, the pathology of 6 cases of left bundle branch block is described. The block was apparent early in life in 2 cases and in one of these the left bundle branch was atretic whereas in the other it was encased in dense fibrous tissue. Two further cases, one with acute block and the other with chronic block were both associated with ischaemic heart disease. The fifth case, with chronic idiopathic left bundle branch block, had focal fibrosis of the proximal left bundle branch. The last case was a patient with sarcoidosis in whom the left bundle branch was severely involved with granulomatous inflammation. It is possible, at autopsy, in the vast majority of individuals with left bundle branch block to recognize an interrupting lesion, provided a careful examination is performed.