Objective: To evaluate factors affecting semi-rigid ureteroscopy (URS) results highlighting the influence of teaching on its outcomes.Material and methods: We reviewed the files of 891 adult patients who had undergone 1182 ureteroscopies at our institute during the period from July 2008 to June 2011. The outcomes of all URSs were evaluated. Outcomes were measured by stone- free rate and presence of complications, which were assessed using the Clavien-Dindo system. Patients were divided into 2 groups; Group 1 (favorable outcome) became stone- free after the first URS and had no documented complications, while Group 2 (unfavorable outcome) had residual stones and/or complications. Group 2 was subdivided according to the skill level of the operating surgeon into two subgroups. Patients belonging to subgroup A had their procedures performed by urology trainees under direct supervision of expert urologists, while those in subgroup B had their procedures performed by the expert urologists themselves. All groups were compared using univariate (chi-square and t tests) and multivariate (logistic regression) statistical tests to identify significant risk factors. All data was analyzed using SPSS.Results: A total of 1182 URSs were evaluated. 958 patients had a favorable outcome (Group 1) while 224 patients had an unfavorable outcome (Group 2). Factors associated with an unfavorable outcome include location of the presenting stone (p<0.001) and presence of stone impaction (p<0.001). No statistically significant differences were detected in the overall complication rate between trainees and expert urologists. Trainees stone- free rate was comparable to that of experts; 90.3% vs. 91.1%, respectively, p=0.6.Conclusion: Factors such as stone impaction and proximal location are associated with an unfavorable surgical outcome. In a high- volume teaching hospital, semi-rigid URS done by trainees under direct supervision is safe and their outcome is comparable to literature findings