Background: Cholangiocarcinoma (CCA) is the most frequent biliary malignancy. It is difficult to diagnose owing to its anatomic location, growth patterns and lack of definite diagnostic criteria. Its cells express and secrete Insulin-like growth factor I (IGF-I). Objective: Measurement of IGF-I in bile and serum of patients with extrahepatic CCA and its evaluation as a diagnostic marker. Methods: The study is a cross sectional study including 60 patients presented to endoscopic retrograde cholangiopancreatography (ERCP) unit for extrahepatic bilairy obstruction. Patients were grouped as follows: Group A including 20 patients with extrahepatic CCA, Group B 20 patients with pancreatic cancer and Group C 20 patients with benign biliary lesions. Results: Biliary IGF-I was significantly different among the studied groups (p=0.030, AUC =1, and at cut-off of 314.92 ng/ml, the sensitivity and the specificity was 100%). The mean biliary IGF-I level, in patients with extrahepatic CCA, was 14 fold higher than in those with pancreatic cancer (p < 0.001), and 19 fold higher than in those with benign biliary lesions (p < 0.001), while it was comparable among patients with pancreatic cancer and benign biliary lesions (p = 0.13). On the other hand, the mean serum IGF-I was comparable among the studied patients, only showing statistical difference between patients with pancreatic cancer and those with benign bilary lesions (p=0.004). Conclusion: Biliary IGF-I is a useful reliable diagnostic test in differentiating extrahepatic CCA from other causes of biliary obstruction.