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A retrospective analysis of 71 patients with proven common bile duct obstruction, who had not undergone previous biliary surgery, was performed. To determine optimal criteria for differentiating stone from nonstone obstruction by ultrasonography, five sonographic signs were analyzed as binary variables. These signs are the demonstration of (1) common duct stone, (2) mass at the site of the common bile duct, (3) gallbladder stones, (4) gallbladder volume greater than a critical value, and (5) common duct diameter greater than a critical value. Maximum expected accuracy was 85% achieved by prediction based on a set of three variables: common duct stone, common duct mass, and gallbladder stones. This was not significantly better than the accuracy (82%) achieved by prediction on the basis of gallbladder stones alone. Three signs in combination, ie, demonstration of gallbladder stones, gallbladder volume less than 52 ml, and common duct diameter less than 14 mm, had predictive value equal to demonstration of a common duct stone as an indicator that common bile duct obstruction is the result of a stone.