The clinical applications of flexible ureterorenoscopes have fallen behind those of their rigid counterparts. Flexible endoscopes that possess a working channel and can also be employed for treatment have at last been developed. This instrument is particularly effective in the treatment of stones in the upper urinary tract, as it can easily follow mobile upper ureteral stones and calculi in the middle or lower renal calices, which are not as accessible with a rigid endoscope. It is important to remember not to exert too much force when inserting the instrument into the ureter, renal pelvis, or renal calices, and, if one remains calm, should the visual field become temporarily obscured, there should be little danger of damaging these structures. This method will probably be increasingly employed, not only for the treatment of stones in the upper urinary tract, but also for the removal of foreign bodies and the diagnosis and treatment of small pathologic conditions or lesions such as carcinoma in situ in which the roentgenologic presentation is unclear. For this expansion of instrument use to be ensured, it is necessary to improve the resolution of the fiberoptic endoscope image and increase the instrument's angulation capabilities, increase the range of accessories that can be employed with it, and also increase familiarity with the instruments on the part of urologists.