Medullary sponge kidney is not a new clinical and pathologic entity. It has been described by European authors since 1939 (12) but only recently has it received attention in the English literature. It has also been called cystic disease of the renal pyramids (15, 21), multiple cysts of the renal medulla (10), cystic dilatation of the renal collecting tubules (22), renal tubular ectasia (19), and simply sponge kidney (3, 5, 17, 20). Lenarduzzi in 1939 (12) first used the term medullary sponge kidney, likening the appearance of the medulla to a sponge. Description The pathologic changes are confined to the medullary portions of the kidney and consist of dilatation of the collecting tubules of the pyramids associated with multiple small cysts and cyst-like cavities. Some of these cystic structures have been shown to communicate with the ectatic tubules, and the dilated, sometimes branched collecting tubules and small cystic structures frequently contain small calculi. More commonly, both kidneys are involved...