Repetitive renal malformation is formed by embryonic developmental variation of the kidney, with an incidence of about 1/125 in the population, mostly combined with complete or incomplete repetitive ureter, and all seven cases in this group were incomplete repetitive ureter. The disease is asymptomatic in about 60% of patients, and the corresponding clinical symptoms are manifested when it is complicated by urinary tract infection, hydronephrosis, urinary stones, and ectopic ureteral opening. As the patient’s superior kidney is often dysplastic, it can easily lead to obstructive pelvic ureteral effusion. Repeated kidney complicating hydronephrosis mostly occurs in the superior pelvic ureter, usually with the main manifestation of swelling and pain in the affected side of the waist, or no obvious symptoms when the degree of effusion is mild. The diagnosis mainly relies on ultrasound, intravenous urography, CT or CTU and other imaging examinations. Ultrasound examination has the advantages of being economical, non-invasive, easy to operate and dynamic observation, and it can show the duplicated renal pelvis and ureter, as well as the complicated hydronephrosis and dilated and tortuous ureter. However, when hydronephrosis is obvious and the renal cortex is thin, it is easy to be confused with renal cysts and adrenal cysts, resulting in misdiagnosis. In our group of 7 patients with duplicated kidney complicated by hydronephrosis, 3 cases were clearly diagnosed by ultrasound, and 4 cases had ultrasound examination suggesting the presence of possible suprarenal pole cyst or hydronephrosis. This suggests to us that for cases with clinical suspicion of duplicated renal malformation, the clinical proposed diagnosis should not be completely rejected by ultrasound examination alone, and should be ruled out by repeated ultrasound examination or another examination. Some scholars reported that the use of diuretic ultrasound to dynamically observe the changes of hydronephrosis can also improve the accuracy of diagnosis. Intravenous urography is a common and reliable method to diagnose hydronephrosis complicated by duplicated kidney, especially high-dose intravenous urography, most of which can directly show the collecting system and ureter of duplicated kidney. However, it is difficult to differentiate intravenous urography from giant renal cyst when the duplicated kidney is complicated by severe superior hydronephrosis and the superior kidney is poorly functioning without visualization or poorly visualized. If the following signs appear on the intravenous urography, it indicates that the duplicated kidney malformation is complicated by hydronephrosis: (1) soft tissue mass above the inferior renal pelvis; (2) the axis of the inferior renal pelvis and calyces is vertical or turned downward, like a low-hanging “lily”; (3) the number of the visible calyces is less than normal; (4) the inferior renal pelvis is far from the upper pole of the kidney; (5) the inferior The ureter of the renal pelvis was displaced laterally. Among the 7 patients in this group, 4 cases were diagnosed by intravenous urography, and 3 cases were not diagnosed clearly because the ureter was not visualized, but showed occupational manifestations such as pushing and shifting of the renal calyces. Therefore, for cases with the above-mentioned features, the diagnosis of hydronephrosis or renal cyst should not be satisfied only, but should be combined with clinical manifestations and other imaging examinations to exclude the possibility of duplicated renal malformation complicated by hydronephrosis in the upper part of the kidney. When the suspected duplicated kidney is complicated by hydronephrosis and the diagnosis is difficult to be confirmed by ultrasound and intravenous urography, CT examination is also an important diagnostic method. Enhanced CT scans often show the duplicated pelvis and calyces and ureter more accurately than ultrasound. The upper renal pelvis is often seen to be hypoplastic and medial, while the lower renal pelvis is normally developed with large and small calyces and is low and lateral. In cases where the superior renal pelvis is cystically dilated due to fluid accumulation, care should be taken not to confuse it with an upper renal pole cyst, which can be identified by looking for duplicated ureteral images. In this regard, CTU 3D imaging technique can show complete images of the kidney and ureter throughout, which is more helpful for diagnosis. In our three cases, the ureter was not visualized on intravenous urography, and the diagnosis was confirmed in two cases after CTU examination, but there was still one case of severe hydronephrosis, which was difficult to be identified with a large renal cyst after all the above imaging examinations. In terms of treatment, mild hydronephrosis with duplicated kidney can be observed and followed up, and no special treatment is needed if there is no significant change in hydronephrosis; for cases with combined urinary tract infection and urinary stones, the corresponding comorbidities can be treated. In cases with severe hydronephrosis, partial nephrectomy should be performed. In this group, three cases of partial nephrectomy were performed and no recurrence of hydronephrosis was seen in 1-3 years of follow-up. Therefore, the diagnosis of hydronephrosis complicated by duplicated kidney malformation often requires the combination of multiple imaging methods to complement each other and make a comprehensive judgment. Partial nephrectomy should be performed for duplicated kidney complicated by severe hydronephrosis. On the other hand, for cases with clinically proposed renal cysts, especially upper pole cysts, the possibility of repeat kidney complicated by severe hydronephrosis should be considered, and corresponding examinations should be performed. Before excluding repeat kidney complicated by hydronephrosis, treatment such as percutaneous puncture injection of anhydrous ethanol should be performed with caution. If the preoperative diagnosis is not clear, the bottom of the cystic cavity should be carefully explored intraoperatively, and if the cystic cavity is connected with the collecting system, partial nephrectomy should be performed for the diagnosis of duplicated kidney complicated with hydronephrosis.