Abstract 1. 1. An electrocardiographic diagnosis of bilateral bundle branch block can be made in rare instances in which conduction in the branches is not completely interrupted and the degree of block in one of the branches is less than in the other on some occasions and greater on others; this allows the patterns of right and left bundle branch block to appear alternately or intermittently in the same patient, accompanied by changes of P-R. Two personal cases of this sort are reported and seven similar observations from the literature are discussed. 2. 2. While the incidence of bilateral bundle branch block in which this condition can be definitely proved is very small, the great incidence of unilateral bundle branch block makes it probable that many cases of complete A-V block, attributed to a conduction disturbance in the A-V node or the common stem of the bundle, are in reality caused by bilateral bundle branch block. 3. 3. In seven of the nine cases of definite bilateral bundle branch block, alternation of the right and left bundle branch block patterns was present at some time. The most probable explanation of this was considered to be 2:1 block in one of the branches in the presence of a constant prolongation of conduction in the other branch. 4. 4. Conduction in the branches behaves in the same way as in the rest of the A-V conduction system. In one of the cases (No. 1), all the possible forms of impaired conduction were demonstrated to be present in the left branch of the His bundle (simple prolonged conduction time, Wenckebach periods, 2:1 and 3:1 block, and transient complete interruption of conduction). 5. 5. True bilateral bundle branch block must necessarily prolong the A-V conduction time. The form of the ventricular complex is determined by the branch with the greater degree of block, while the conduction delay in the branch in which the block is less important determines the lengthening of the P-R interval. This must be so because the stimulus reaches the ventricules through the less affected branch. 6. 6. In true bilateral bundle branch block the intrinsicoid deflection of the ventricular complex is delayed only on the side of the chest corresponding to the more affected branch and is within normal limits on the opposite side. Cases in which the intrinsicoid deflection is delayed over both ventricles probably represent an association of true bundle branch block with an intraparietal conduction disturbance of the contralateral ventricle.