Choledochal cysts are congenital anomalies of the biliary tract manifested by cystic dilatation of the extrahepatic and intrahepatic bile ducts. Choledochal cyst is not rare in far-East Asian countries. Type II choledochal cysts account for 2% of all such cysts. They are true diverticula of the extrahepatic bile duct and communicate with the bile duct through a narrow stalk. This condition is associated with significant complications, such as ductal strictures, stone formation, cholangitis, rupture and secondary biliary cirrhosis. We describe a case of a huge impacted stone in a diverticular choledochal cyst which masqueraded as an unusual cystic duct stone causing Mirizzi's syndrome.