Two new cases of exercise related left-bundle branch block are reported in this study. They should be added to the twenty established and published cases. Comparison opens the way to discussion of the mechanism of onset. Ischemia is often accused, although no exploration has been able to demonstrate its responsibility in the origin of the problem. These two cases have novel features, particularly in describing the first case of painful exercise related bundle branch block in a subject twenty-three years of age. Ischemia linked to epicardial coronary lesions cannot be held responsible, but the possibility of micro-circulatory ischemia is suggested.