Background <p>BK virus (BKV), JC virus, and simian virus 40 (SV40) are polyomaviruses with high genome identity. Therefore, SV40 staining is used for immunostaining to detect polyomaviruses such as BKV. BKV reactivation has been reported to be a risk for high-grade bladder cancer in immunosuppressed kidney transplant patients. We report a kidney transplant patient with SV40 staining-positive muscle invasive bladder cancer.</p> Case presentation <p>The patient was a 74-year-old woman who received a kidney transplantation in China 17&#xa0;years prior. After the age of 71 years, decoy cells were detected in urine cytology about once a year, there was no gross hematuria or urinary tract abnormality, and her graft kidney function was stable. However, at the age of 73, urine cytology was suspicious for malignancy, and cystoscopy revealed a bladder tumor. Transurethral resection of the bladder tumor was performed, and pathology revealed high-grade muscle invasive bladder cancer with micropapillary variant and SV40-stained cells in the lesion. Immediate radical cystectomy was performed, and pathology revealed invasive urothelial carcinoma pT3aN0M0 with positive ureteral resection margins. An 18 F-fluorodeoxyglucose (FDG)-positron emission tomography (FDG-PET)-computed tomography (CT) after cystectomy showed peritoneal dissemination not detected by contrast-enhanced CT. A total of four courses of systemic chemotherapy with gemcitabine-cisplatin were administered, but the patient died of cancer 6&#xa0;months after the diagnosis of bladder cancer.</p> Conclusions <p>Proactive screening of kidney transplant patients with decoy cells is important because of the possibility of high-grade bladder cancer associated with BKV, even in the absence of hematuria. FDG-PET/CT is useful for accurate staging of high-grade urothelial carcinoma in kidney transplant patients.</p>