Introduction: Ureteral colic is a common painful disorder. Early surgical intervention is an attractive management option but existing evidence does not clarify which patients benefit. Based on lack of evidence, current national specialty guidelines provide conflicting recommendations regarding who is a candidate for early intervention. We compared treatment failure rates in patients receiving early intervention to those in patients offered spontaneous passage to identify subgroups that benefit from early intervention. Methods: We used administrative data and structured chart review to study consecutive patients attending one of nine hospitals in two provinces with an index emergency department (ED) visit and a confirmed 2.0-9.9 mm ureteral stone. We described patient, stone and treatment variables, and used multivariable regression to identify factors associated with treatment failure, defined as the need for rescue intervention or hospitalization within 60 days. Our secondary outcome was ED revisit rate. Results: Overall, 1168 (37.9%) of 3081 eligible patients underwent early intervention. Patients with small stones <5mm experienced more treatment failures (31.5% v. 9.9%) and more ED revisits (38.5% v. 19.7%) with early intervention than with spontaneous passage. Patients with large stones ≥7.0mm experienced fewer treatment failures (34.7% v. 58.6%) and similar ED revisit rates with early intervention. Patients with intermediate-sized 5.0-6.9mm stones had fewer treatment failures with intervention (37.4% v. 55.5%), but only if stones were in the proximal or middle ureter. Conclusion: This study clarifies stone characteristics that identify patients likely to benefit from early intervention. We recommend low-risk patients with uncomplicated stones <5mm generally undergo initial trial of spontaneous passage, while high-risk patients with proximal or middle stones >5mm, or any stone >7mm, be offered early intervention.