A 3-year-old boy was referred to us with a diagnosis of bladder tumor. On cystoscopy, a yellowish-white pedunculated tumor was found at the right trigone and biopsy revealed an inflammatory change. Computed tomographic (CT) scan showed a soft tissue density from the lower ureter to the bladder. Open surgery confirmed a ureteral polyp originating from the lower ureter prolapsing into the bladder, which mimicked bladder tumor. The need of open surgery in such cases is discussed.