Wolff-Parkinson-White syndrome type A obscured by left bundle branch block

2 patients are presented whom the presence of left bundel branch block obscured the signs of the Wolff--Parkison--White syndrome expected with a bypass on the left side of the heart. While it was possible to infer the presence of both disorders from the surface ECGS, electrophysiological studies proved invaluable in defining the precise situation in both cases. When atypical electrocardiographic appearances are observed in the Wolff-Parkinson-White syndrome, careful investigation may reveal the cause to be bundle branch block on the same side of the hear as the anomalous pathway. In both patients there was in addition evidence of sinoatrial disease, which was associated with the occurrence of arrythmias, and this was also confirmed with intracardiac studies.