Increased heart rate variability (HRV) is a condition that is usually manifested during electrocardiogram (ECG) examinations due to its own abnormalities. It is an indicator for evaluating autonomic function as well as cardiac and humoral function, and an increase in HRV is a predictor of fatal cardiac diseases, such as sudden cardiac death (SCD) or malignant arrhythmia, and is an early guide for diseases that are caused by autonomic function, such as diabetes and hyperthyroidism. In addition, it is also useful for the early prediction of diabetes mellitus and hyperthyroidism due to autonomic nervous system function. Therefore, patients with markedly increased heart rate variability should be examined promptly to rule out the possibility of fatal heart disease and other autonomic disorders in the light of their own performance, and should be reviewed regularly.