A case of sponge kidney associated with pyelon e phritis arisen in 39 years old man was reported. Discussions were made on its patho-histological findings and clinical pictures. 1. Histological characteristics of sponge kidney are localized cystic dilatati on of the collecting ductus in the renal medulla and papillae. Glomerular and interstitial lesions often associated with the above abnormalities are not considered to be the essential change sponge kidney but are thought to be the secondary alterations. Interrelationship and difference between the cystic dilatation and cystic-forming disease of the kidney or calyceal diverticula should be further investigated. 2. Diagnosis of the disease can be relatively easily made with characteristic X-ray picture of the renal pelvis, despite presence of complications often maze confirmation of the disease to be extremely difficult. 3. The disease is often a ccompanied with renal calcinosis or renal calculi and is also the cause of these abnormalities or inflammatory renal diseases. Therefore differential diagnosis with primary hyperparathyroidism, primary renal acidosis, renal inflammations, severe alkalosis due to gastric or duodenal ulcers and others must be carefully made with the references of results of laboratory examinations, such as various renal function tests, serum electrolytes, serum alkaline phosphatase activity, blood pH values, blood CO2 capacity and urinary excretion of Ca and P. 4. Our case had a chief compla int of pyuria without association of vesical symptoms and fever, and showed fairly normal results on various renal function tests. However histological examination of the kidney demonstrated marked changes due to nephritis, interstitial nephritis and pyelitis as well as intracystic calcifications. These changes are thought to be reversible at this stage, so that an appropriate treatment can possibly avoid critical figures. A long term follow-up study is essential in order to take precautions against development of renal calculi and renal function disturbances.