Urinary tract infections (UTIs) represent a significant public health concern worldwide, accounting for approximately 40% of all antibiotic prescriptions. Acute pyelonephritis is an infectious-inflammatory renal disease primarily affecting the pelvicalyceal system and the tubulointerstitial tissue. In Russia, this condition is diagnosed in over a million individuals annually, predominantly among young and middle-aged women. Notably, the incidence shows a seasonal pattern, with peaks during the summer months. In 2025, the European Association of Urology revised its classification of UTIs. The revised framework eliminates the distinction between "uncomplicated" and "complicated" UTIs and instead delineates between localized and systemic UTIs based on clinical signs and symptoms. Importantly, the updated classification no longer identifies males as a risk factor for these infections. The principal pathogens responsible for acute pyelonephritis include Escherichia coli (identified in 70–95% of cases), Klebsiella spp., Proteus mirabilis, Enterobacter spp., Pseudomonas aeruginosa. Microbial entry into the kidneys may occur through an ascending route or via hematogenous spread. This article examines the predisposing factors, etiology, pathogenesis, and clinical manifestations associated with the disease. Detailed discussions are provided regarding diagnostic stages, from the assessment of complaints to the application of instrumental methods. Additionally, pertinent questions for patient medical history indicative of acute pyelonephritis are specified. Significant emphasis is placed on antibacterial therapy, with the preferred agents being fluoroquinolones and cephalosporins, as well as aminoglycosides, and, in regions exhibiting high resistance, carbapenems. The article presents empirical treatment regimens for both oral and parenteral therapies, in accordance with the guidelines of the 2026 European Association of Urology and the Ministry of Health of Russia. Specific treatment strategies for pregnant women and patients with chronic kidney disease are also discussed. Acute pyelonephritis is a prevalent condition amongst urological patients, and if not addressed promptly, it has the potential to result in disability. However, with prompt diagnosis and the appropriate selection of therapeutic strategies, the prognosis for this disease remains favorable.