Transurethral lithotripsy was performed in 63 patients with ureteral calculi between September 1987 and March 1991. Three types of flexible nephroureteroscopes with diameters of 12.8F, 10.5F and 9.3F were used in groups 1, 2 and 3, respectively. The rate of successful stone disintegration and the stone-free rate 3 months after lithotripsy were 73% (11 of 15 patients) and 67% (10 of 15), respectively, in group 1, 93% (26 of 28) and 82% (23 of 28), respectively, in group 2, and 95% (19 of 20) and 85% (17 of 20), respectively, in group 3. Four complications were encountered in the patients treated with the 12.8F device (group 1). Our results clearly show that use of a slimmer fiberscope and the effect of learning to perform techniques with the flexible nephroureteroscope improved the success rate and decreased the complication rate in transurethral lithotripsy.