Aim: To present a patient with congenital bilateral hydronephrosis and the consequent unilateral afunctional kidney as one of the rare developmental anomalies of the urotract, and to point out the importance of using ultrasound for the purpose of early detection and prevention of further irreversible damage to the renal parenchyma. Case report: A male newborn was admitted to the Department of Intensive Care and Neonatology due to prenatally diagnosed bilateral hydronephrosis in the 26th week of gestation. During the stay, an ultrasound of the kidneys was performed, which showed hydronephrosis of the IV degree on the right, with dilated calyces and ballooned pyelon, and slightly reduced parenchyma. At the expected location of the left kidney, an irregular, bag-like formation was shown in the expected area of the duct system, while the kidney parenchyma was not discernible. At the age of 1 month, a dynamic scintigraphy of the kidneys was performed, which showed slow drainage from the right kidney in the area of the P-U junction. The left kidney was not shown during the examination. Given the described finding, the patient was treated with conservative methods. The boy was of regular growth and development. At the age of two years, a control scintigraphy was performed, which again showed significantly slowed spontaneous drainage in the area of the P-U junction, without improvement with the application of diuretics. Given that the findings deteriorated, the patient underwent surgical treatment. The method of choice was internal drainage with a “double J” prosthesis. This was followed by outpatient monitoring of renal function parameters. Conclusion: Hydronephrosis can be treated by double J surgical placement and thus potentially avoids more invasive methods of surgical treatment. However, it is necessary to approach each patient individually, taking into account the indications and possible complications.