(1) U-loop vectorcardiograms were obtained from 53 patients by the method of differential vectorcardiography. The subjects were composed of 11 cases of right ventricular hypertrophy, 21 cases of right bundle branch block and 21 cases of myocardial infarction.(2) The U loop of right ventricular hypertrophy and of complete or incomplete right bundle branch block resembled that of normal subjects. Namely, it had a shape like a small, slightly curved club and was inscribed in the direction of continuance of the terminal limb of the T loop. A few cases of right bundle branch block showed a definite exception to this statement.(3) The U loop of myocardial infarction began with a marked bend at the T-U junction and extended in various directions. It resembled also a small curved club, but was sometimes much larger than normal U loops. Namely, it resembled that of left ventricular hypertrophy. But the U loop abnormality in myocardial infarction was usually more marked than in left ventricular hypertrophy.(4) In reviewing the U loop abnormality in ventricular hypertrophy, bundle branch block and myocardial infarction at least 2 factors could be thought of to cause the U loop abnormality: hypertension and organic changes in the myocardium.