The patient with gallstones has an unknown risk of also having choledocholithiasis. To establish this risk preoperatively, several tests were studied in 167 patients, 37 of whom had common duct stones at exploration. With no previous or current jaundice, as well as normal alkaline phosphatase levels and a duct size of less than 12 mm, there was less than a 5% risk of choledocholithiasis; elevated bilirubin and/or alkaline phosphatase levels indicated intermediate risks (33% to 66%); and a duct size of 12 mm or greater, with any combination of other factors, indicated a 90% to 100% risk of choledocholithiasis. The surgeon should be aware of these probabilities in evaluating conditions of patients with cholelithiasis.