BACKGROUND: The ureteral access sheath (UAS) is an ancillary device widely used by urologists, but acute ureter injury may occur following its insertion. Preoperative selective oral alpha1-blockers can reduce intraureteral pressure, and prevent ureteral wall injury during UAS insertion. OBJECTIVE: To compare perioperative data of patients who underwent flexible ureterorenoscopy (fURS) with UAS with and without premedication with silodosin. DESIGN, SETTING, AND PARTICIPANTS: Single-blind, 100 patients from a single institution who underwent retrograde intrarenal surgery for kidney and upper ureter stone removal were prospectively allocated from May 2018 to March 2019. INTERVENTION: The experimental groups received silodosin for 3 d preoperatively. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: The primary endpoint included ureteral injuries after UAS insertion that were assessed according to endoscopic classification. The secondary endpoint was an evaluation of whether premedication with silodosin had any effect on postoperative outcomes. RESULTS AND LIMITATIONS: A total of 44 and 43 patients were randomly assigned to the control and experimental groups, respectively. Silodosin prevented significant postoperative ureteral injury involving the smooth muscle layer more successfully than in the control group (9.3% vs 27.3%; p = 0.031). There was no significant difference in the overall complication rate as determined by the modified Clavien-Dindo classification system and the computed tomography scan stone-free rate postoperatively. Patients who received silodosin before fURS reported lower pain scores than those in the control group using a visual analog scale (p = 0.009). Limitation included a lack of placebo comparison. CONCLUSIONS: Our data suggest that preoperative silodosin protects against significant ureteral injury related to UAS insertion during fURS and decreases postoperative pain level. Silodosin premedication might be an effective and safe technique to replace prestenting. PATIENT SUMMARY: We investigated the preventive effect of an alpha-blocker against perioperative complication caused by ureteral access sheath inserted during flexible ureterorenoscopy. Taking silodosin before surgery prevented ureter wall injury during surgery and immediately improved postoperative pain.