Purpose: To study the efficacy and safety of the Dornier MPL 9000 lithotripter in the treatment of prevesical calculi using real-time ultrasound monitoring and to see if efficacy is adequately judged by the efficiency quotient (EQ). Patients and Methods: Seventy-six patients underwent ultrasound-guided extracorporeal shockwave lithotripsy (SWL) for prevesical calculi over a period of 43 months. Their age ranged from 8 to 68 (mean 37.6) years, and the male:female ratio was 2.6:1. The size of the stones, measured in two dimensions, ranged from 4. to 25 mm (mean 9.3 mm) and 2 to 15 mm (mean 6.0 mm). No general or regional anesthesia or ureteral stents were used, and all patients were treated in the prone position. The EQ was calculated using the formula: Stone free (%) x 100/(100 + retreatment rate (%) + auxiliary procedures (%). Results: Seventy patients were stone free in a mean time of 15 ± 14.11 days. The average number of shockwaves used was 2831 ± 1612, and the average number of sessions per patient was 1.7. About 92% of the patients were rendered stone free using in situ SWL alone. No major complication was encountered, and none of the patients required an inpatient stay after SWL. The EQ was 65. Five patients with failed SWL subsequently were rendered stone free, four with salvage ureteroscopy and pneumatic lithotripsy and one with cystolitholapaxy for a symptomatic fragment in the bladder. One patient was lost to follow-up. Conclusion: In situ SWL is a safe and effective treatment for prevesical calculi. It should be used as a first-line treatment for most such stones. Efficiency can be assessed objectively by the EQ.