To evaluate the safety and effectiveness of ureteroscopic stone manipulation, we retrospectively reviewed the failure and complication rates of two series of retrograde ureteroscopies (URS) according to the instrument used: the conventional rigid ureteroscopes and the new fiberoptic small size ureteroscopes. From July 1985 to December 1992, we performed 248 URS in 238 patients using a conventional rigid ureteroscope. From January 1992 to December 1992, we performed 49 URS in 47 patients using a semirigid ureteroscope. The failure rate was 14.5% in the first group of patients. The early and late complications observed were: urinary tract infections (UTI) in 22.5%, small mucosal lesions (false route) in 24.5%, full-thickness perforations in 11.2%, migration of the stone fragments into the retroperitoneum in 0.4% and the rupture of the tip of an alligator forceps into the ureteral lumen in 0.4%. Ureteral stenosis was observed in 1.6% of the cases. Often two or more complications occurred in the same patient. In the second group the failure rate was 10.2% and the complication rate 8.1%. We reported one case of ureteral perforation (2%), 3 mucosal lesions (6.1%) and UTI in 2 cases (4%). We conclude that URS using a small-caliber semirigid ureteroscope is a safer procedure than URS with conventional instruments. The majority of failures with semirigid scopes is related to the laser ineffectiveness in fragmenting the stone.