The objective of this study was to evaluate the performances of the Storz Modulith SL20 lithotriptor. Fifty patients with a total of 52 renal (31) or ureteric (21) stones were treated, in a single session, between June and October 1993. The mean stone diameter was 7.9 mm. All patients were reviewed after 3 months. The complete success rate, with radiological cure, was 65% at 3 months. The partial success rate, defined as fragmentation of the stone with persistence of residual fragments less than 3 mm in diameter, not requiring further treatment, was 12% at 3 months. The failure rate at 3 months was 23%. The commonest complication was renal colic in 12 patients (24%). Two patients developed an extrarenal haematoma. The Modulith SL20 possesses a good detection system: firing is well tolerated under minimal analgesia. Our success rates are slightly lower than those obtained by other teams using the same apparatus.