The aim of the present study was to assess the value of EUS in identifying the cause of CBD diltation which its nature could not be diagnosed by transabdominal ultrasonography and other modalities as computed tomography (CT). This prospective longitudinal study was conducted on 50 adult patients between March 2018 and March 2019 because of dilated CBD of undetrmined etiology on TUS and CT. Thirty-eight patients (76%) were males and twelve patients (24%) were females. Patients were selected according to the following criteria: Patient with dilated CBD > 7 mm, with no definite cause of biliary obstruction on abdominal sonography and CT and patients with dilated biliary system and suspected periampullary lesions or enlarged pancreatic head with no definite focal masses which sent for more 
clarification by EUS. The diagnostic performance of: US, CT, and EUS in prediction of malignancy showed that abdominal US had sensitivity of (57.1%) and specificity (100%) while, CT had sensitivity of (68.6%) and specificity of (100%) while EUS had sensitivity of (85.7%) and specificity of (100%).