Introduction: Retroperitoneal tumor is often seen in urology clinical practice. To diagnose the tumor, tumor specimens must be obtained. However, in some cases, the tumor is penetrated by vessels around the ureter, and it may be difficult to detect the optimal spot for obtaining a specimen, even when performing open surgery. Case Presentation: A 53-year-old male patient was referred to our hospital for further examination of left back pain due to hydronephrosis. Enhanced computed tomography demonstrated ureter stenosis in front of the ilium, which was surrounded by a retroperitoneal tumor. The tumor was penetrated by blood vessels
therefore, we performed an open surgical biopsy on the suspicion of a retroperitoneal tumor using ureteroscopic assistance. The diagnosis of idiopathic retroperitoneal fibrosis was made according to the biopsy. Conclusion: We herein report the first case of a ureteroscopy-assisted biopsy for the pathological diagnosis of a retroperitoneal tumor.