Background: Upper tract urothelial carcinoma (UTUC) is an uncommon disease that is diagnosed clinically by the selective use of urine cytology, urine biomarkers, and imaging of the upper tract. We present a case of a patient with Lynch syndrome and high-grade UTUC that was diagnosed by an abnormal Cxbladder assay, prompting further endoscopic examination. Case Presentation: A 59-year-old Caucasian female with a history of endometrial cancer and bladder cancer with Lynch syndrome presented for evaluation of recurrent urothelial carcinoma. Her previous bladder tumors have been T1 high grade and Ta high grade and have been treated with resection and multiple cycles of intravesical Bacillus Calmette-Guerin (BCG) therapy. She had also undergone a robotic left distal ureterectomy and psoas hitch for a high-grade distal ureteral tumor. Surveillance cystoscopy 7 months after revealed a biopsy-confirmed bladder tumor, which was resected, and she was started on maintenance BCG therapy. At presentation, follow-up urine cytology and UroVysion studies were negative. Cxbladder test was also initially negative. However, during close clinical monitoring, the Cxbladder test became positive. Cystoscopy was once more performed, which was unremarkable. Bilateral ureteroscopy was performed, revealing high-grade upper tract renal papillary carcinoma (UTUC) in the left renal pelvis. The patient declined a nephroureterectomy. She was treated with two sessions of holmium laser ablation of the left renal pelvis tumor and underwent 6 weekly courses of BCG + interferon instilled into her left renal pelvis using a 5F open-ended catheter. Repeat urine cytology, UroVysion, and Cxbladder tests were negative after completion of upper tract BCG therapy. Conclusion: Cxbladder test may be useful and an adjunct to urine cytology and the UroVysion FISH assay to evaluate patients at high risk for recurrent UTUC.