New generation flexible ureterorenoscopes offer an improved deflection mechanism and a stiffer sheath compared to their predecessors. We aimed to determine if these improvements in design lead to a higher efficacy in the treatment of nephrolithiasis. Ninety patients with upper urinary tract calculi were included into a retrospective analysis. Twenty-nine cases were treated with the conventional flexible ureterorenoscope (11274 AA, Karl Storz Endoscopy, Germany) and 61 cases were treated with the new generation device (Flex-X, Karl Storz Endoscopy). Patients' and stone characteristics, intraoperative data, treatment success and complications were retrieved from the charts and compared between the two groups. Preoperative data were comparable in both groups. Whereas stone access was also comparable (97 vs. 100%; n.s.), immediate treatment success was significantly higher for the new flexible scope (70 vs. 38%; p = 0.003). For the subgroup of lower pole stones, stone access was possible in 94 versus 100% (n.s.) and treatment success was 31 versus 69% (p = 0.0004) for the conventional and the new generation device, respectively. No major complications were observed, minor complications were comparable in both groups. Our study suggests an advantage of the new generation flexible ureterorenoscopes compared to their predecessors. They offer an increased stone free rate especially in the treatment of lower pole stones. It seems therefore advisable to switch to the latest generation flexible devices.