We reported a 4-year-old girl with history of bilateral vesicoureteral reflux who underwent direct radionuclide cystography in our department. Radiotracer was mistakenly injected in the catheter balloon. The filling phase showed no change in the bladder volume, and the catheter balloon was apparent as an intense activity in the base of the bladder. The procedure was repeated with correct injection of the tracer into the catheter. Wrong injection of the radiotracer into the catheter balloon should always be borne in mind in similar cases.