To analyze the predictive factors for retreatment in RIRS to achieve complete lithiasis resolution.Retrospective comparative study analyzing 298 cases of RIRS performed in our center over a 3 year period. The cohort was divided in two groups: Resolution in one operation or more than one, evaluating homogeneicity for age and gender. We compared the folowing variables: Hounsfield units, body mass index (BMI), number of stones, size, (on the case of multiple stones, larger stone size), side, location in the kidney and stone biochemistry. Bivariant statistical analysis by Student's t and Chi square tests, and multivariate analysis by binary logistic regression. ROC curves were made to set cutting points for relationship between quantitative variables.The groups were homogeneous for both age and gender (p>0.05). 260 (87.25%) patients required one treatment only and 38 (12.75%) more than one. Among the study variables, the only one that showed differences between the groups wa stone size, being the mean size 18 mm in the single treatment group and 26 mm in the more than one treatment group (Difference between mean values -8.27, 95%CI: -5,91 - -10.63, p<0,001).RIRS with holmium laser is still an effective technique for the treatment of renal lithiasis. The largest stone size is related with the need of retreatments, so it must be taken into consideration specially over 2 cm. In our series, for every extra millimeter in size the probability of retreatment increased 1.14 times, demonstrating the importance of size in this context.