Purpose: We performed a randomized, prospective, multi-institutional study evaluating the durability of commercially available flexible ureteroscopes. Materials and Methods: A total of 192 patients were randomized to the use of 7 less than 9Fr flexible ureteroscopes, including the Storz™ 11274AA and Flex-X™, the ACMI™ DUR™-8 and DUR™-8 Elite, Wolf™ models 7330.170 and 7325.172, and the Olympus™ URF-P3. Information about total and lower pole use time, the number and method of ureteroscope insertion, and they type and duration of accessory instrumentation was recorded. Surgeons were asked to rate the visibility and maneuverability of the instrument on a scale of 0-poor to 10-excellent. Results: The indication for ureteroscopy was upper tract calculi in 87% of cases. Of ureteroscope insertions 97% were performed through an access sheath. The average of number of cases before repair ranged from 3.25 for the Wolf™ 7325 to 14.4 for the ACMI™ DUR™-8 Elite. Average ureteroscope operative time was statistically longer for the DUR™-8 Elite (494 minutes) than for the Flex-X™ (p = 0.047), and the Wolf™ 7325 and 7330 (p = 0.001 and 0.001, respectively). Duration of use before repair for the URF-P3 (373 minutes) was statistically longer than for the Wolf™ 7325 and 7330 (p = 0.016 and 0.017, respectively). Minutes of use with an instrument in the working channel were significantly more with the DUR™-8 Elite and the URF-P3 than the Wolf™ 7330 (p = 0.017 and 0.008) and 7325 (p = 0.012 and 0.005, respectively). The ureteroscope that experienced the greatest average duration of lower pole use was the URF-P3, while the shortest was the Wolf™ 7325 (103 vs 20 minutes, p = 0.005). Average minutes of laser use before breakage was significantly longer for the DUR™-8 Elite than for the Wolf™ 7325 (110 vs 21 minutes, p = 0.021) and 7330 (24 minutes, p = 0.025). Conclusions: Currently available less than 9Fr flexible ureteroscopes remain fragile instruments. The DUR™-8 Elite and Olympus™ URF-P3 proved to be the most durable devices.