Fourteen patients with transient bundle-branch block have been personally studied. All but one suffered from ischemic heart disease commonly accompanied by hypertension. The conduction defect was observed to appear during acute infarction, attacks of prolonged chest pain, and episodes of left ventricular failure. Not infrequently, however, the appearance and disappearance of bundle-branch block was unaccompanied by any recognizable change in the patient's physical condition. Bundle-branch block may revert to normal intraventricular conduction after many years. In one patient bundle-branch block disappeared on three separate occasions after having been present for over 12 months. Another patient is described in whom normal conduction returned after left bundle-branch block had been established for a record duration of 6 years. The etiology, prognosis, and pathogenesis of transient bundle-branch block have been discussed and the practical importance of this conduction defect in the electrocardiographic diagnosis of myocardial infarction is mentioned.