ABSTRACT— HEPAR is an expert system which can be used as a supportive tool in the diagnosis of disorders of the liver and biliary tract. When consulted for a particular patient, the system assesses the patient's disorder in two stages. In the first stage, data of medical history, physical examination and routine laboratory tests are used to determine whether the disorder is hepatocellular or biliary obstructive, and whether benign or malignant features are present. In the second stage, the system produces a differential diagnosis out of a set of 80 disease categories, using the conclusions of the first stage of the assessment and the results of a restricted number of supplementary investigations, i.e. serological tests and ultrasonography. The conclusions of the two stages are ordered by the amount of evidence computed by HEPAR. The system is not yet suitable for clinical use. The system has been evaluated using data of 181 patients selected from a population of 214 consecutively admitted patients with jaundice. The patients were classified by the system as having a hepatocellular or biliary obstructive, and a benign or malignant disorder in 96% and 100% of the cases, respectively. When comparing the conclusions with strongest evidence with the final clinical diagnoses, the patients were classified correctly in 86% and 83% of the classified cases, respectively. A differential diagnosis was produced in 87% of the patients. A correspondence between the diagnosis with strongest evidence and the final clinical diagnosis was found in 78% of the classified patients.