Extracorporal shock wave lithotripsy has rapidly become established world wide as a routine method for treatment of nephro-and ureterolithiasis. Although initial studies showed no tissue damaging effect by the shock waves, we found in an animal experiment using canine kidneys, the ESWL induced damage to the renal parenchyma is more marked than originally assumed. The damage is limited to the area that was focused on, and heals relatively rapidly by connective tissue encapsulation with final cicatrisation without any further residual effects being observed until now. This parenchymal damage is probably also the cause of the macrohaematuria that is always observed during therapy. The resulting tissue damage is not extensive enough to cause demonstrable reduction of function as measured by the usual methods (serum creatinine, creatinine clearance, isotopy renography, i/v-urography). The main clinical complication is the large subcapsular haematoma which, according to present knowledge, could well result from a lesion of the larger peripheral vessels. Damage to other organs such as subserous colonic and small bowel haematomata are to be expected although they do not lead to clinical symptoms.