Residual urine, also called residual urine volume, refers to the amount of urine left in the bladder after urination. In a normal person, the residual urine is about 15 ml. If it is more than 15 ml, it is called increased residual urine volume. In clinical practice, patients can be asked to measure the residual urine volume with ultrasound immediately after urination. The residual urine volume can be used as one of the indicators to diagnose the disease, or to determine the effectiveness of the treatment after the disease has been treated. Increased residual urine is often the result of lower urinary tract obstruction, such as prostatic hyperplasia or prostate cancer in middle-aged and elderly men, urethral trauma, urethral stricture, bladder neck obstruction, and neurogenic bladder caused by traumatic brain injury, spinal cord trauma, diabetes mellitus, and cerebrovascular disease.