This prospective observational outcome study assessed the impact of helical computed tomography (CT) scan in patients with a first episode of suspected nephrolithiasis. Before CT scanning, Emergency Physicians completed a questionnaire, including diagnostic certainty of nephrolithiasis and anticipated patient disposition. Primary outcome measure was the comparison of physician diagnostic certainty and CT scan results. Secondary outcome measures included alternate diagnoses and changes in patient disposition after CT scan. Four categories grouped the pre-CT diagnostic certainty: 0-49%, 50-74%, 75-90%, and 90-100%. The CT scan found urinary calculi in 28.6%, 45.7%, 74.2%, and 80.5% of patients in each category, respectively. CT scanning revealed alternate diagnoses in 40 cases (33.1%). Of these, 19 (47.5%) included other significant pathology. Before CT scanning, physicians planned to discharge 115 patients and admit six patients. After CT scanning, six of the former group were admitted, and five of the latter group were discharged. Patients presenting with a first episode of clinically suspected nephrolithiasis should undergo CT scanning because it enhances diagnostic certainty by identifying alternate diagnoses not suspected on clinical grounds alone.