PURPOSE: To investigate change of CT attenuation of biliary stones in water soluble contrast media with time as well as the factors contributing to this change. MATERIALS AND METHODS: Thirty biliary stones were placed within cone-shaped plastic tubes, and as a control study, spiral CT scanning was performed 50 minutes after immersion in normal saline. The stones were rescanned at 5, 10, 20, 30, 45, 60, 90 and 120 minutes after immersion in water soluble contrast media. Mean CT attenuation value and volume of the stones were measured after three-dimensional reconstruction of images. Physical factors such as porosity[(wet weight - dry weight) / wetweight]x100, volume, and cholesterol as a chemical factor were measured. RESULTS: The pattern of change of CT attenuation was classified as one of three types. Fifteen stones (50%) were classified as type 1, in which attenuation increased with immersion time; in ten stones, this increase was rapid, particularly within the first 5 minutes. Twelve (40%) were classified as type 2, in which attenuation showed no significant change. Three stones showed no regular pattern, and these were classified as type 3. The mean porosity of type 1 (median; 32.7, mean±SD; 52.83±34.48) was greater than that of type 2 (median; 6.7, mean±SD; 30.58±48.25)(p<.05). The volume and cholesterol fraction of stones were not significantly different between type 1 and 2. CONCLUSION: In some biliary stones, CT attenuation value increases in water-soluble contrast media with time, and porosity is the most important factor in attenuation change.