UNFORTUNATELY, not all operations on the biliary system are successful. When the surgeon fails to remove all offending intraductal calculi, the "residual" stones often provoke undesirable complications. The frequency with which secondary operations are employed is forceful testimony to such failures. We, too, have been embarrassed by this particular problem. Occasionally our primary operation has failed to relieve the intraductal obstruction, and the patient continued to suffer from the stasis of bile. How frequently do calculi remain in the common bile duct after gall-bladder surgery? Does the clinical course after operation furnish reliable evidence of the presence of residual choledochal stones? Why are the elusive calculi missed at the primary operation? Can the frequency of residual biliary calculi be reduced? What treatment should be given to patients with these residual stones? With the cooperation of the radiology departments in 11 hospitals in our area, we have conducted a study with the purpose of obtaining factual