A 42-year-old woman had presented to the local clinic with jaundice of two months duration and she was suspected of having periampullary cancer; she was then referred to us and admitted to our hospital for further evaluation. The physical examination revealed no abnormality. The serum total bilirubin level was 3.1 mg/dl and the aspartate aminotransferase/alanine aminotransferase levels were 80/180 U/L. Abdominal CT showed a cystic dilatation of the common hepatic duct and a marked dilatation of the intrahepatic bile duct (Figs. 1A, B). Abdominal ultrasonography also revealed these lesions, but there was a partial thin septum-like structure within the upper aspect of the cystic dilated common hepatic duct (Fig. 1C). However, neither definite septation nor an enhancing portion within the dilated duct was noted, and so this was considered to be a choledochal cyst. Percutaneous transhepatic biliary drainage (PTBD) was done through the left hepatic duct, and the subsequent tubogram showed obstruction of the left hepatic duct by a large filling defect. On endoscopic retrograde cholangiopancreatography (ERCP), a well-demarcated filling defect was seen in the common hepatic duct and this did not communicate with the bile duct. The distal portion of the common hepatic duct showed a beaking appearance near this mass and the left hepatic duct was not opacified (Fig. 1D). Any anomalous pancreaticobiliary ductal union was not found. Before surgery, magnetic resonance cholangiopancreatography (MRCP) was performed, and it showed a well-defined cystic mass extending from the left hepatic duct to the common hepatic duct with a definite thin wall displaying a dark signal intensity. Magnetic resonance cholangiopancreatography also showed a bile duct variation; the right posterior segmental duct drained into the left hepatic duct (Fig. 1E). The patient underwent resection of the extrahepatic biliary tract including the cystic mass, and hepaticojejunostomy was next performed. During surgery, the cystic mass was noted to arise from the common hepatic duct, yet it had no communication with this duct. Pathologically, this unilocular cystic mass arose from the serosal surface of the common hepatic duct, which was bulging into the common hepatic duct (Fig. 1F). It was lined by a single layer of tall columnar mucin-secreting cells with focal evidence of mucinous secretion (Fig. 1G). Our case was diagnosed as a biliary mucinous cystadenoma in the common hepatic duct. The patient has been alive with no tumor recurrence for one year after surgery.