Abstract 1. 1. Four cases having electrocardiograms with the following features have been presented: (a) signs of left bundle branch block in the limb leads; (b) signs of right bundle branch block in the precordial leads; (c) qR deflections in aVR. 2. 2. Vectorcardiograms were obtained in these cases, all of which reveal the nature of the conduction defect to be left bundle branch block. 3. 3. The mechanisms responsible for the unusual vectorcardiographic and electrocardiographic features have been discussed. 4. 4. These observations suggest that electrocardiograms showing the paradoxical features described be interpreted as left bundle branch block with extensive septal and inferolateral myocardial infarction. 5. 5. As might be expected in posterior infarction, high grade atrioventricular block is an occasional feature.