Purpose Unenhanced helical computerized tomography (CT) has proved to be an excellent diagnostic tool for evaluating acute flank pain with reported 95% to 100% sensitivity, 92% to 100% specificity, 96% to 100% positive and 91% to 100% negative predictive values. The diagnostic value of a new low dose protocol was prospectively studied and compared with the results of conventional unenhanced helical CT in a previous series with an effective dose equivalent (HE) of 3.1 to 4.3 mSv. and in current literature with an estimated HE of 4.3 to 4.7 mSv. Materials and Methods In 109 patients 18 to 86 years old with acute flank pain we performed low dose unenhanced helical CT in addition to abdominal ultrasound and urinalysis with new CT parameters (120 kV. 70 mA., 5 mm. collimation, pitch 2 and incremental reconstruction each 5 mm.) that led to a more than 50% decrease in radiation exposure to 1.50 mSv. in females and 0.98 mSv. in males. Ureteral calculi were confirmed or excluded by retrograde ureteropyelography in 51 cases. In the other cases the diagnosis was verified by the clinical and ultrasound course, and/or stone asservation. Results In 80 of the 109 patients the flank pain was caused by a ureteral calculus. Low dose unenhanced helical CT precisely identified 77 ureteral calculi with 1 false-positive finding. Thus, the sensitivity and specificity of low dose unenhanced helical CT were 96% and 97% with a 99% positive and 90% negative predictive value. In 15 of 29 patients with CT findings negative for stone disease different causes of pain were established by low dose unenhanced helical CT. Conclusions Even with the significantly decreased radiation exposure of the low dose protocol unenhanced helical CT is still an excellent and rapid diagnostic tool for evaluating acute flank pain with lower radiation exposure than excretory urography (HE 1.3 to 2.3 mSv.) at our departments. Only in obese patients with a body mass index of greater than 31 kg./m. 2 is conventional unenhanced helical CT with higher radiation exposure recommended to achieve adequate image quality.