Erythropoietin (EPO) serum levels were measured in 30 patients before, during and immediately after shock wave treatment by an enzyme immunoassay. Shock wave treatment was performed in the Dornier lithotripter HM3 under a constant voltage of 18 kV. In 16 out of 30 patients, an increase in the EPO serum level of over 10 U/l more than the value obtained prior to the treatment was observed. Differences ranged from 11 to 50 U/l. In 3 patients EPO serum concentration after shock wave treatment rose above the physiological limit of 54 U/l. Increased EPO serum levels have been observed after applications both under and over 1,000 shock waves. The small number of cases does not allow a statistical correlation between the number of shock waves and the elevation of the EPO serum concentration. Further investigations are necessary to define the circumstances by which an increase in EPO serum level occurs under shock wave treatment.