The new generation semiflexible ureterorenoscopes (URS) offer an improved deflection of 270 in both directions and a stiffer sheath. It is postulated that stone access and removal are facilitated compared to the conventional devices. In our study, we aimed to determine the difference in efficacy of stone treatment comparing the new and the conventional flexible URS. A total of 90 patients with upper urinary tract stones were included into a retrospective study. 29 cases were treated with the last generation flexible URS (11274 AA, Karl Storz Endoscopy, Germany) between 01/2000 and 07/2003 and 61 cases were treated with the new flexible URS (Flex-X, Karl Storz Endoscopy. Germany) between 12/2005 and 05/2007. The patients underwent standard clinical and radiological work-up before the procedure. Patients' and stone characteristics, stone free rate and complications were compared between the two groups. Treatment success was defined as complete stone removal or insignificant residual fragments <4mm requiring no further intervention. Stone access was possible in 97% and 100% for the conventional and new URS group. Immediate treatment success was 38% for the conventional versus 70% for the new flexible scope (p=0.0032). For the subgroup of lower pole stones mean stone size was 6.6 mm (conventional scope, 16 patients) versus 5.3 trim (new scope, 51 patients). Stone access was possible in 94% versus 100% and treatment success was 31% versus 69% (p=0.0004) for the old compared to the new scope. No major complications were observed. Minor complications were comparable in both groups. Our study suggests an advantage of the new flexible ureterorenoscopes compared to their predecessors. They offer an increased stone free rate especially in the treatment of lower pole stones.