Presence of stone-like deposits of substances hard to dissolve in urine in the renal capsule or renal pelvis. Most of the stones are made up of calcium oxalate and calcium phosphate crystals. Others consist mainly of uric acid or cystine, an amino acid. Often the cause of stone formation is unknown; sometimes it is increased production of the substances named. Calcium, for example, is overproduced in cases of excess corticosteroid hormone and osteoporosis, uric acid in gout and cases of malignant tumour. Excess cystine in the urine usually results from a congenital condition. Other possible factors are limited fluid intake or excessive moisture loss by sweating or diarrhoea, causing higher urine concentration. Local damage caused by inflammation or congenital malformation provides ideal places for the formation of deposits which can grow into stones. Urine acidity is also a factor; urine stones are formed only in very acid urine, and may be dissolved again when acidity is reduced. Discomfort caused by kidney stones varies with their position and size. Stones in the kidney itself often produce no symptoms. This is also the case with small stones and ‘gravel’ which can be passed easily in the urine. Discomfort begins if a stone sticks in a ureter and causes renal colic4’, associated with severe pain in the kidney area and abdomen, radiating to the groin. Sometimes blockage by a stone can cause hydronephrosis, and damage to the mucous membrane of the urinary tract by a stone can induce haematuria (blood in the urine). Bacteria can easily establish themselves in a stone, thus leading to inflammation. Further general symptoms are nausea and vomiting. Treatment is by drinking large quantities of fluid, if necessary supplemented with diuretics. Some stones can be dissolved, others can be removed by the insertion of a special instrument into the urethra (cystoscope), or broken down so that they can be passed in the urine. Sometimes surgery is needed to remove the stone. The most modern treatment is the painless breaking down of stones by high-frequency shock waves administered while the patient is immersed in a bath of water.