Diagnosing the jaundiced patient whose disease fails to become typical of any one disease even after a month of clinical observation can be an extremely difficult task because there are six diseases which mimic each other very closely: nonmalignant biliary obstruction, chronic active hepatitis, primary biliary cirrhosis, drug-induced jaundice, malignant extrahepatic obstruction and cholestatic viral hepatitis. In the diagnosis of the etiology of jaundice, the most important distinction to make is between jaundice that requires surgical intervention for its relief and that which requires medical treatment. At best, noncomputer diagnostic methods are able to make the medical versus surgical diagnosis with somewhat less than 90 per cent accuracy. The computer diagnostic system described in this study was able to make the correct medical versus surgical diagnosis in fifty-two cross validated test patients who had prolonged undifferentiating liver diseases with 98.1 per cent accuracy. It correctly identified the specific disease causing the patient's jaundice, the exact diagnosis, in fifty-two cross validated test patients with 77.0 per cent accuracy. The system was able to make the correct medical versus surgical and exact diagnosis by simple reclassification with 100 per cent accuracy. © 1969.