With the advent of chemical dissolution and lithotripsy for the treatment of gallstones, use of imaging techniques to determine the chemical composition of gallstones has become important. In vitro CT scans were performed on gallstones removed at surgery from 70 patients. The CT appearances were as follows: dense (20%), faint (10%), isodense with saline (nonvisualized) (26%), rimmed (30%), and laminated (14%). Stones from 50 patients were subjected to chemical analysis, and the results were compared with the appearance of the stones on CT. Among stones grouped by CT appearance, a significant difference in cholesterol content was found, but calcium content did not differ significantly among most groups. The inverse correlation between CT attenuation values and cholesterol was stronger (r = -.82) than the correlation between CT attenuation values and calcium (r = .64). The results show that gallstones can be divided into those with high cholesterol content and those with low cholesterol content on the basis of their appearance on in vitro CT. The differences in the CT findings in many gallstones appear to relate more to cholesterol content than to calcium composition.