Recent developments of fiberoptic technology have resulted in significant miniaturization of existing flexible fiber-scopes. Instruments now exist with external diameters that approach those of the catheters that are used for percutaneous nephrostomy or for nephrostomy track dilation. We report our initial experience with a prototype tip-deflecting fiberscope that is small enough to accommodate guidewires, small catheters, and cytology brushes, and that can be manipulated into calyces, down the ureter, and occasionally even into the bladder. The small external diameter of this nephroscope and its flexibility allow it to be inserted over a guidewire through an ordinary percutaneous nephrostomy track. Its overall applicability was somewhat limited by its small channel size. Development of an instrument of similar external dimensions with a channel large enough to accommodate stone baskets, forceps of various types, and electrodes, seems well worthwhile.