The incidence of carcinoma in choledochal cyst is much higher than that of normal population. The mechanism of carcinogenesis in choledochal cyst is not clear, though reflux of pancreatic juice and stasis of bile appears to be important factors. To study the clinical characteristics of carcinoma in choledochal cyst and to present the rationale that resection is the choice of surgery in choledochal cyst, we reviewed 8 cases of carcinoma in choledochal cyst. The risk of carcinoma in choledochal cyst was 13.1%(GB cancer; 2 cases, CBD cancer; 5 cases, periampullary cancer; 1 case). The sex ratio(M:F= l: 1) and mean age(41 yrs) were intermediate between choledochal cyst(l:3, 36 yrs) and other bile duct cancer(2:1, 59 yrs). All patients had abdominal pain and 4 cases had weight loss which was rare in patients with choledochal cyst only. Five cases were type I and 3 cases were type IVa by Todani type. Anomalous pancreaticobiliary ductal union could be confirmed in 3 cases. Radical operations were possible in one case of GB cancer who received cholecystectomy and cyst excision, and in 2 cases of common bile duct cancer who received Whipples operation. Four patients who had previous history of biliary operation 1 month to 4 years before, received only palliative operation. The patient with periampullary cancer discharged without operation due to multiple liver metastasis. All patients expired between 3 months to 13 months after diagnosis of cancer except 2 patients: one patient who underwent cholecystectomy and cyst excision due to early GB cancer, has been alive for 5 years and one patient who underwent pailliative cystojejunostomy has been alive with disease for 3 months. In conclusion, the risk of carcinoma in choledochal cyst is very high and the prognosis is very poor, so the possibility of cancer association should always be considered and eariy excisional therapy is recommended for those who are diagnosed as choledochal cyst.