Choledochal cyst is a congenital malformation of the biliary tree that is unusual but by no means rare. During a 13-year period, we encountered eight patients with choledochal cysts who were evaluated with cholangiopancreatography. All of our cases showed an anomalous union of the pancreatic duct and common bile duct, resulting in a long common channel. Review of cholangiopancreatograms also showed ectasia of the common channel in six of the eight patients. Mean length of the common channel, corrected for magnification, was 26 mm (normal, <15 mm). The mean corrected diameter of the common channel was 7 mm (normal, 3-5 mm). We conclude that ectasia of the common channel is an important additional radiographic observation in the diagnosis of choledochal cyst. This observation has not been emphasized before.