The diagnostic value of unenhanced helical computed tomography was investigated in a prospective study. In 53 patients (aged 35 to 82 years) with acute flank pain tomography was performed in addition to abdominal plain film and ultrasound examination. All 53 patients had a contraindication for intravenous administration of contrast medium. Ureteral calculi were either confirmed or excluded by retrograde ureteropyelography in 44 cases, in 9 patients by asservation of calculi and clinical follow-up. Helical computed tomography was able to precisely identify all of the 34 ureteral calculi, whereas abdominal plain films led to 6 false positive and 17 false negative findings. In 1 patient with retroperitoneal lymphoma (diagnosed by CT) false positive findings occurred. Unenhanced helical computed tomography reaches a distinctively increased diagnostic value (sensitivity 100%, specificity 95%, accuracy 97%) in the evaluation of acute flank pain as compared to conventional radiologic imaging and ultrasound. This non-invasive procedure is to be considered method of choice for patients with contraindications for the application of radiopaque material.