OBJECTIVES The objectives of this prospective study are to present our experience with retrograde intrarenal surgery (RIRS), and to analyze its results and complications. MATERIAL AND METHODS 150 patients with renal stones were treated with RIRS. 111 cases showed single stones whilst multiple stones were observed in 39 cases. The mean size was 19.12mm (r: 5-74). Success rate was defined as the absence of residual stones or the presence of fragments ≤2mm. RESULTS in 21 (14%) patients RIRS could not be performed on first attempt because it was impossible to place the ureteral access sheath. The immediate success rate was 85.7%, and 91.6% at three months later. The average operating time was 85min (r: 25-220). Postoperative complications were observed in 22 cases (14.6%), although most of them were classified as Clavien 1 and 2 (19 cases), and only 2% (3 cases) showed Clavien 4 complications (sepsis requiring admission in the intensive care unit). 10 patients underwent a second procedure in order to complete the treatment. Thus, the number of procedures per patient was 1.06. There were no late complications. CONCLUSIONS the treatment of renal stones with flexible ureteroscopy using the ureteral access sheath shows a high successful rate with a low complication rate. In order to define its indication more precisely, randomized studies comparing RIRS with minimally invasive percutaneous nephrolithotomy procedures (miniperc and microperc) would be necessary.