Pyelonephritis is an infection in the upper urinary tract of children. It is caused by infectious factors and the exact localization of the microbial inflammatory process is not always clear. It can also affect the lower urinary tract, causing infections such as cystitis and urethritis. The infection can be acute or chronic and can spread easily. The prevalence of pyelonephritis among children is higher in boys under 1 year of age. In different regions, the prevalence varies, with rates in Russia, western Europe, and the USA ranging from 4-27.5%.