Background: Many etiologies can be behind common bile duct dilatation that is undiagnosed, such as pancreatic and periampullary tumors, choledocholithiasis, or an inflammatory condition as stenosis. Endoscopic ultrasound (EUS) may have a major diagnostic value in detecting etiologies of dilated CBD undetected by other non-invasive modalities. Objectives: This study aimed to determine whether endoscopic ultrasonography is helpful in detecting a common bile duct dilatation that isn't clear. Methods: This study was conducted on 67 participants who had unexplained CBD dilatation at Tropical Medicine Department, Zagazig University. Every patient underwent TUS, CT, MRCP and EUS. Findings of EUS were seen and verified by ERCP, surgery, or a biopsy that revealed cancer. Those who had normal results underwent three months of clinical monitoring. Results: 77.6% and 59.7% of patients underwent US and CT respectively prior to MRCP and EUS. By MRCP, 82.1% of patients were normal, 11.9% had stone, while 3% had CBD mass and 3% had stricture. While by using EUS, 50.7% were normal, 22.4% had stone, while 8.9% had ampullary lesion. Post EUS outcome 37 (55.2%) of patients had only medical follow up while 22.4% had stone extracted by ERCP, 6 patients (8.9%) had sphincterotomy and biopsy from lesions, 4 cases of them had adenocarcinoma, the other 2 cases had adenoma and 2 cases had CBD stricture treated by sphincterotomy and stent dilatation. 2 cases of pancreatic cancer underwent Whipple operation. EUS denied results of MRCP in 25 cases. Conclusion: EUS has a very useful role in the diagnostic work up for patients with unexplained CBD dilatation with or without symptoms.