The aim of our study was to compare the accuracy of non-contrast spiral CT with ultrasound (US) for the diagnosis of ureteral calculi in the evaluation of patients with acute flank pain. 62 consecutive patients with flank pain were examined with both CT and US over a period of 9 months. All patients were prospectively defined as either positive or negative for ureterolithiasis, based on follow-up evaluation. 43 of the 62 patients were confirmed as having ureteral calculi based on stone recovery or urological interventions. US showed 93% sensitivity and 95% specificity in the diagnosis of ureterolithiasis; CT showed 91% and 95%, respectively. Pathology unrelated to urinary stone disease was demonstrated in six patients. Although both modalities were excellent for detecting ureteral stones, consideration of cost and radiation lead us to suggest that US be employed first and CT be reserved for when US is unavailable or non-diagnostic.