The evolution of flexible ureteroscopes led to a widespread use for the management of upper urinary tract abnormalities. The cost of purchase, maintenance and the durability of these instruments has become a major issue. This work describes a new device to avoid damages due to incorrect use of the Holmium laser during insertion of the laser fibre. A laser fibre with an optical core of 271 and 430 muicrom outside diameter was slid inside the FlexGuard laser fibre insertion sheath (LISA laser products, Germany). The outside diameter of the sheath measures 2.7 F (0.9 mm) and 2.1 F (0.7 mm) luminal diameter. The distal fibre tip was brought up to a position app. 2 mm inside the distal end of the sheath. The loaded sheath was pushed through the working channel of various ureteroscopes which were in maximum deflection. With the insertion sheath protruding about 2 mm from the distal tip of the URS the fibre was effortless forwarded out of the sheath to approach the stone. Once the laser fibre was in position, the sheath was removed, to increase the volume of irrigation fluid during laser lithotripsy. The radius of curvature (ROC) of the URS in maximum deflection and the integrity of the working channel was investigated. Using the insertion sheath the laser fibre reached the working position without any recognition of scratching or resistance. The integrity of the ureterorenoscopes was checked thoroughly be manually operated manometry. No damage of the inner surface of the working channel occurred. The ROC of the instrument did not change significantly during this procedure. After removal of the sheath the ROC remained stable. With the extended use of ureteroscopy, durability and repair costs are of concern. Damage resulting from incorrect use of laser fibres is a major issue in this respect. FlexGuard proved to avoid this damage in all flexible ureteroscopes investigated without limiting their mobility.