The authors describe a 23-year-old man who showed colonic activity during renal scintigraphy. He had a ureterosigmoidostomy performed 5 years earlier because of congenital bladder exstrophy. Results of routine biochemical tests were normal. Dynamic renal scintigraphy with Tc-99m MAG3 revealed good renal function except for bilateral transient pelvicaliceal retention of radioactivity. Intense accumulation was noted in the colonic area that increased in time and extended to the descending and transverse colon, findings that were secondary to the ureterosigmoidostomy.