"Medullary sponge kidney" applies to pathologically dilated collecting tubules within one or more renal pyramids of one or both kidneys, almost always diagnosed radiographically, of uncertain etiology, and presenting a clinical spectrum varying from an asymptomatic, incidental finding to severely complicating calcareous-infective disease, renal insufficiency, and death. Recognition of the characteristic urographic pattern affords the patient presenting clinically with hematuria, ureteral colic, urinary tract infection, or nephrocalcinosis a prompt diagnosis with a frequently benign prognosis, and usually averts more extensive or invasive investigations.