1. 1. U-loop vectorcardiograms were obtained from 30 normal subjects and from 31 subjects with left ventricular hypertrophy, by the method of differential vectorcardiography. Four of the subjects with left ventricular hypertrophy were excluded from the present study because of the complication of myocardial infarction. 2. 2. The U loop of normal subjects showed a constant configuration. It resembled a small, slightly curved club and was inscribed almost in the direction of continuation of the terminal limb of the T loop, so that a term such as U vector segment or U vector arc might be preferable to the term U vector loop. The T-U junction vector was directed to the left, inferiorly, and mostly anteriorly. This finding of the U loop, together with that of the T loop, which was inscribed clockwise in the frontal plane and counterclockwise in the horizontal and left sagittal planes, with its long axis directed to the left, inferiorly, and slightly posteriorly or anteriorly, constituted characteristic features of the normal T-U loop. 3. 3. The U loop of left ventricular hypertrophy began with a marked bend at the T-U junction and extended in various directions, resembling also a small curved club, but sometimes much larger than normal U loops. Occasionally, the U loop was inscribed as in normal cases, in continuation of the terminal limb of the T loop, but in such cases the T loop itself was abnormal. All cases of left ventricular hypertrophy showed such a feature of the T-U loop in at least one planar projection; there was one exception, however, a case in which normal features were retained in all three planes. 4. 4. There was a tendency toward relatively slight left ventricular hypertrophy in those cases which retained normal features in any one of the planes or those which showed abnormality in the U loop only, whereas there was relatively marked left ventricular hypertrophy in those cases which showed a distinct abnormality also in the T loop. 5. 5. Such U-loop abnormalities were already evident in the early stage of left ventricular hypertrophy when there were no (or minute) evidences of it in ordinary electrocardiograms, including the U-wave changes. Thus, they are useful for diagnostic purposes in detecting slight left ventricular hypertrophy.