Abstract 1. 1. In 1000 cases with diagnostic electrocardiographic patterns, 94 cases of abnormal U waves were found. The U-wave patterns in these 94 cases showed the following chief characteristics: 1.1. (a) In left heart strain and in intraventricular block of the common type the U-wave abnormalities consist of inverted U waves in Leads II and CF 4. 1.2. (b) In anterior wall infarction abnormal (inverted or diphasic) U waves are found in Leads CF 2 and CF 4 with occasional similar abnormality in Lead I. 1.3. (c) In posterior wall infarction inverted U waves are found most commonly in Lead CF 4. 1.4. (d) No abnormal U waves were found in right heart strain or in intraventricular block of the indeterminate S type. 2. 2. A hitherto undescribed U-wave abnormality, a negative bowing of the T-P segment in Lead I, is discussed. 3. 3. Instances of distortion of the T and P waves by large upright U waves and the confusion caused by the presence of prominent U waves in records of auricular fibrillation are pointed out. 4. 4. Evidence is presented relating the presence of abnormal U waves to abnormalities of the S-T-T segment. In left heart strain and in the common type of intraventricular block, there is usually concordancy in the direction of the T wave, S-T deviation and U wave direction. In posterior wall infarction there is concordancy of the deviation of the S-T segment and the direction of the U wave but discordancy between T and U. In anterior wall infarction the reverse is true. 5. 5. In serial records the abnormal U wave may undergo an evolution independent of the changes in the S-T-T segment.