It is known that in pyelonephritis, E. coli is often isolated from the urine. However, in recent years, along with this classic pathogen, patients with pyelonephritis began to isolate other uromicroflora. Thus, a number of studies have reported an increasing role of Proteus and Pseudomonas aeruginosa in the etiology of pyelonephritis in children and adults. A characteristic tendency to change the bacterial microflora depending on the duration of the disease, intensity and method of treatment has also been noted. According to some authors, the change of microorganisms may occur both during instrumental examinations and surgical interventions on the urinary system organs, and under the influence of antibacterial drugs. A number of studies have shown an increasing and even leading role of staphylococcus aureus in the etiology of some forms of pyelonephritis in children. There is an opinion that staphylococcus represents the main danger as an infectant in bladder catheterization and urinary tract drainage operations. Thus, further study of the microbial etiology of pyelonephritis is an urgent task in pediatric urology.