Objective: The aim of the study was to determine if the degree of hydronephrosis on focused emergency renal ultrasound correlates with kidney stone size on computed tomography. Methods: A retrospective study was performed on all adult patients in the emergency department who had a focused emergency renal ultrasound and ureterolithiasis on noncontrast computed tomography. Severity of hydronephrosis was determined by the performing physician. Ureteral stone size was grouped into 5 mm or less and larger than 5 mm based on likelihood of spontaneous passage. Results: One hundred seventy-seven ultrasound scans were performed on patients with ureteral calculi. When dichotomized using test characteristic analysis, patients with none or mild hydronephrosis (72.9%) were less likely to have ureteral calculi larger than 5 mm than those with moderate or severe hydronephrosis (12.4% vs 35.4%; P < .001) with a negative predictive value of 0.876 (95% confidence interval, 0.803-0.925). Conclusion: Patients with less severe hydronephrosis were less likely to have larger ureteral calculi.