A basic and clinical study was made on the significance of stagnancy of urinary flow as a factor causing urinary tract infections. A bacterial solution was inoculated into the bladder of untreated and diabetic mice with model ascending pyelonephritis. The longer the external urethral opening was clamped, the lower became the value of ID50, thus showing infection easily caused by urinary stagnancy in the bladder. Patients who had UTI within 6 months after prostatic operation were divided into two groups, one with residual urine and the other without residual urine. Then, the type of the causative organism, and the severity of pyuria and bacteriuria were determined. In the former, there were slightly more patients with infection caused by plural bacteria. However, no other differences were observed. The administration of cephem antibiotics revealed no differences in effectiveness between the two groups. In conclusion, stagnancy of urinary flow has been considered a factor easily causing infection. However, so far as residual urine is concerned, it is not an intractable factor in treatment.