Subcapsular or perirenal bleeding is the most commonly experienced adverse effect directly attributable to externally applied shock waves. The first consecutive 3,620 extracorporeal shock wave lithotripsy treatments with the HM3 Dornier lithotriptor at our institution resulted in 24 hematomas in 21 patients, for an incidence of 0.66 per cent. Various factors associated with treatment were examined. The number of shock waves (up to 2,000) and voltage up to 24 kv. did not correlate with the development of hematoma. Coagulation studies were normal in all patients with hematomas. There was no correlation of patients size and weight, or stone size, number or location with the occurrence rate of perinephric hematoma. Patients with pre-existing hypertension, particularly those with unsatisfactory control of hypertension, had a significantly increased incidence of perinephric hematoma. The incidence of hematoma in hypertensive patients was 2.5 per cent and it increased to 3.8 per cent in patients with unsatisfactory control of hypertension. Therefore, pre-existing hypertension is a significant risk factor in the occurrence of post-extracorporeal shock wave lithotripsy bleeding. The incidence of perinephric hematoma also was increased in patients with pre-treatment urinary tract infection and those who underwent simultaneous bilateral treatment. Management of post-extracorporeal shock wave lithotripsy bleeding generally is conservative although a third of the patients required transfusion.