We observed a 30-year-old woman in whom a large mass in the right lower quadrant and bilaterally enlarged cystic kidneys with markedly distorted collecting systems by excretory urography suggested a diagnosis of polycystic renal disease (PRD); but the linear and cystic dilatation of collecting tubules, many of which contained numerous calculi, was typical of medullary sponge kidney (MSK). Arteriography and CT scanning clearly showed that both the calcifications and the cysts were confined to the medullary portions. The cortical layer was entirely free of cysts except where large medullary cysts protruded to the surface. In patients in whom MSK imitates PRD, these conditions can be distinguished by CT scanning and arteriography.
