© 2018 American Medical Association. All rights reserved. IMPORTANCE Urinary stone disease is a common presentation in the emergency department, and α-adrenergic receptor blockers, such as tamsulosin, are commonly used to facilitate stone passage. OBJECTIVE To determine if tamsulosin promotes the passage of urinary stones within 28 days among emergency department patients. DESIGN, SETTING, AND PARTICIPANTS We conducted a double-blind, placebo-controlled clinical trial from 2008 to 2009 (first phase) and then from 2012 to 2016 (second phase). Participants were followed for 90 days. The first phase was conducted at a single US emergency department; the second phase was conducted at 6 US emergency departments. Adult patients were eligible to participate if they presented with a symptomatic urinary stone in the ureter less than 9 mm in diameter, as demonstrated on computed tomography. INTERVENTIONS Participants were randomized to treatment with either tamsulosin, 0.4 mg, or matching placebo daily for 28 days. MAIN OUTCOMES AND MEASURES The primary outcome was stone passage based on visualization or capture by the study participant by day 28. Secondary outcomes included crossover to open-label tamsulosin, time to stone passage, return to work, use of analgesic medication, hospitalization, surgical intervention, and repeated emergency department visit for urinary stones. RESULTS The mean age of 512 participants randomized to tamsulosin or placebo was 40.6 years (range, 18-74 years), 139 (27.1%) were female, and 110 (22.8%) were nonwhite. The mean (SD) diameter of the urinary stones was 3.8 (1.4) mm. Four hundred ninety-seven patients were evaluated for the primary outcome. Stone passage rates were 50% in the tamsulosin group and 47% in the placebo group (relative risk, 1.05; 95.8% CI, 0.87-1.27; P = .60), a nonsignificant difference. None of the secondary outcomes were significantly different. All analyses were performed according to the intention-to-treat principle, although patients lost to follow-up before stone passage were excluded from the analysis of final outcome. CONCLUSIONS AND RELEVANCE Tamsulosin did not significantly increase the stone passage rate compared with placebo. Our findings do not support the use of tamsulosin for symptomatic urinary stones smaller than 9 mm. Guidelines for medical expulsive therapy for urinary stones may need to be revised.