[en] Computed tomography (CT) was employed in an effort to analyze the chemical composition of urinary calculi. Twenty-three surgically removed calculi were scanned in a water bath (in vitro study). Forteen of them in the body were scanned (in vivo study). The calculi consisted of four types: mixed calcium oxalate and phosphate, mixed calcium carbonate and phosphate, magnesium ammonium phosphate, and uric acid. The in vitro study showed that the mean and maximum CT values of uric acid stones were significantly lower than those of the other three types of stones. This indicated that stones with less than 450 HU are composed of uric acid. In an in vivo study, CT did not help to differentiate the three types of urinary calculi, except for uric acid stones. Regarding the mean CT values, there was no correlation between in vitro and in vivo studies. An experiment with commercially available drugs showed that CT values of urinary calculi were not dependent upon the composition, but dependent upon the density of the calculi. (Namekawa, K.)