The patient, a 50-year-old male, was admitted to the hospital with “left renal pelvis occupancy” due to “left lumbar swelling and pain for more than 20 days”. The patient started to have left lumbar distension and pain after catching a cold more than 20 days ago, which was persistent and relieved by oral painkillers. He consulted a local hospital and underwent retrograde pellucidography of the left renal pelvis and CT of the urinary tract, suggesting an occupying lesion in the left renal pelvis. On examination, there was no bulge in the bilateral renal area and no mass was palpable. Percussion pain in the left renal area (+). No mass was palpable in the bilateral ureteral travel area, no pressure pain, no rebound pain, muscle tension; no bulge in the bladder area, normal percussion boundary, no pressure pain. Color ultrasound (outside hospital): left hydronephrosis with dilatation of the upper ureter of the left kidney. KUB+IVP (outside hospital): delayed visualization of the left renal pelvis and calyces, obvious hydronephrosis and dilatation, and no visualization of the ureter. CT (external hospital): high-density nodule in the left renal pelvis and calyces, considering stone; left renal hilar mass with uneven enhancement. Retrograde imaging of the left renal pelvis (outside hospital): left renal pelvis and lower renal calyces with occupying lesions. Admission diagnosis: left renal pelvis occupancy, left renal pelvis cancer? After admission, further urological CT was performed: left renal pelvis occupying lesion, pelvic carcinoma was possible, and secondary hydronephrosis in the left renal calyx. Radical laparoscopic resection of left renal pelvis cancer was performed in our hospital. Postoperative pathology: inflammatory pseudotumor of the left kidney. The patient was admitted to the hospital with “left renal pelvis occupancy” as the main reason for “left lumbar distension and pain for more than 20 days”. The patient started to have left lumbar distension and pain after catching a cold more than 20 days ago, which was persistent and could be relieved after taking oral painkillers. He consulted a local hospital and underwent retrograde pellucidography of the left renal pelvis and CT of the urinary tract, suggesting an occupying lesion in the left renal pelvis. On examination, there was no bulge in the bilateral renal area and no mass was palpable. Percussion pain in the left renal area (+). No mass was palpable in the bilateral ureteral travel area, no pressure pain, no rebound pain, muscle tension; no bulge in the bladder area, normal percussion boundary, no pressure pain. Color ultrasound (outside hospital): left hydronephrosis with dilatation of the upper ureter of the left kidney. KUB+IVP (outside hospital): delayed visualization of the left renal pelvis and calyces, obvious hydronephrosis and dilatation, and no visualization of the ureter. CT (external hospital): high-density nodule in the left renal pelvis and calyces, considering stone; left renal hilar mass with uneven enhancement. Retrograde imaging of the left renal pelvis (outside hospital): left renal pelvis and lower renal calyces with occupying lesions. Admission diagnosis: left renal pelvis occupancy, left renal pelvis carcinoma? Renal inflammatory pseudotumor, an occupying tumor-like lesion, is actually an inflammatory proliferative lesion. It is more common in lung and liver, and less common in kidney. Roth first reported renal inflammatory pseudotumor in 1980. Etiology: It may be related to infection or autoimmune reaction or recurrent stimulation by renal calculi. Inflammatory pseudotumor pathology: benign limited proliferative inflammatory lesions with a predominance of plasma cells, lymphocytes or histiocytes , preferably in young adults. Clinical manifestations: low back pain, fever, hematuria and lumbar masses, or no symptoms at all. Imaging: The tumor is located in the renal pelvis and renal parenchyma, and may be solitary or multiple, or may develop bilaterally. It can be differentiated from renal tumors, plasmacytoma, yellow granulomatous pyelonephritis, focal bacterial nephritis, etc. The preoperative rate of definitive diagnosis of renal inflammatory pseudotumor (including ultrasound-guided puncture biopsy) is about 5%-20%. Anti-inflammatory treatment is effective: with the combined application of antibiotic treatment, the condition of most patients can be significantly improved within 2 weeks, and the lesions mostly disappear after 2-3 months. Further percutaneous renal arteriography can be performed, and if necessary, multi-point puncture biopsy can be performed under ultrasound guidance. After admission, CT of the urinary tract was performed again: the left renal pelvis was an occupying lesion with a high possibility of renal pelvic cancer and secondary left calyceal effusion. Radical laparoscopic resection of left renal pelvis cancer was performed in our hospital. Postoperative pathology: inflammatory pseudotumor of the left kidney.