In November 1991, a third generation extracorporeal shock wave lithotriptor (Piezolith 2500) was installed in our clinic. We examined the effective and safe aspect of this machine for urolithiasis. From November 1991 to October 1993, we experienced 530 sessions of ESWL treatment on 235 patients (270 cases). All patients except two underwent ESWL treatment without anesthesia, and in situ ESWL treatment was possible on 247 cases. The mean number of shock waves and sessions per patient of renal stone were 3369 and 2.1, respectively. Those of ureteral stone were 3970 and 1.9, respectively. The complete removal rate of renal and ureteral stone 3 months after the last session were 86% and 92%, respectively. This outcome was very satisfactory. On the other hand, endoscopic procedure was needed on 13 patients. In almost all cases hematuria was noted after ESWL, but major complications such as subcapsular hematoma or gastrointestinal hemorrhage were not experienced at all. It was concluded that Piezolith 2500 was very effective and safe in the treatment of urolithiasis.