Deterioration of renal function has been reported after radical cystectomy (RC) with urinary diversion. We investigated renal function changes in elderly bladder cancer (BCa) patients who underwent RC with cutaneous ureterostomy (CU) urinary diversion. We performed a retrospective, observational study. BCa patients aged ≥75 with an American Society of Anesthesiologists (ASA) class greater than II were included. Glomerular filtration rate (GFR) was the main outcome measure. GFR values were recorded preoperatively, at discharge, at 6-month follow-up, and yearly up to 60 months. A total of 70 patients with a median age of 78.0 years were identified. Median preoperative GFR was 74.3 mL/min/1.73 m2 and declined significantly to 54.6 mL/min/1.73 m2 after 6 months (p < 0.001). A gradual GFR decline was observed thereafter, reaching a median value of 46.2 after 60 months. Preoperative GFR and acute kidney injury were significant predictors of fast deterioration of GFR and of 25% deterioration of GFR after 12 months. Elderly BCa patients with high comorbidity rates undergoing RC with CU should be carefully informed about the risk of GFR deterioration and the need for adequate monitoring.