Purpose: To evaluate the usefulness of ultrasonography (US) as a disposition tool in emergency care centers for categorizing urolithiasis patients who present with acute flank pain and microscopic hematuria. Methods: We enrolled patients who presented with acute flank pain and microscopic hematuria from January to August 2006 in the Emergency Care Center of Chungnam National University Hospital. We categorized patients into four groups according to their hydronephrosis and urinary complications, respectively, and retrospectively reviewed their charts relative to patients` disposition. Results: One hundred eighty-two patients met the criteria and were enrolled in the study. Patients who were classified as group D (without complication or hydronephrosis) had 100% negative predictive value for invasive procedures (ureteroscopy or double J catheterization), hospital admission, extracoporeal shock wave lithotripsy (ESWL), and prescription of antibiotics. Conclusion: It is reasonable for emergency department residents to route patients without complication or hydronephrosis to the outpatient department after pain control and simple therapeutic education.