Introduction: Choledochal cysts are rare anomalies of the bile duct that generally present in childhood but can rarely be delayed until adulthood in 20% of cases. They are often diagnosed incidentally or present with acute pancreatitis or cholangitis. We report a case of a type 1 choledochal cyst (T1CC) and type C anomalous pancreaticobiliary maljunction (PBM), successfully managed by a multidisciplinary approach. Case Description/Methods: A 52-year-old woman presented with right upper quadrant pain, jaundice and vomiting. Labs revealed elevated lipase, bilirubin, AST, ALT and alkaline phosphatase. A CT of the abdomen showed no gallstones but a marked dilation of extrahepatic bile duct with a transition point in the distal intrapancreatic portion (Figure a). An endoscopic retrograde cholangiopancreaticogram (ERCP) showed fusiform dilation of the CBD consistent with T1CC (Figure b). Spyglass cholangioscopy revealed focal stenosis in the distal CBD joining above the ventral pancreatic duct opening within the long common channel, suggestive of type C anomalous PBM (Figure c). A plastic biliary stent was placed into the CBD. EUS guided fine needle biopsy of the stricture showed no evidence of malignancy. Her clinical course was complicated by multiple admissions due to stent occlusion. After multidisciplinary discussion, the patient underwent choledochal cyst resection with hepaticojejunostomy, cholecystectomy and biliary reconstruction. Intraoperative gallbladder specimen revealed poorly differentiated adenocarcinoma with plan for chemotherapy. Discussion: Choledochal cysts are a rare congenital anomaly characterized by enlargement of the CBD. There are five types of choledochal cysts (Table). Etiology remains unclear and while some believe these cysts are congenital in nature, others propose they arise from anomalous PBM. PBM is defined as congenital malformation in which the pancreatic and bile duct join outside the duodenal wall forming a long common channel. This leads to reflux of biliary secretions leading to chronic inflammation, increasing the risk of developing cholangiocarcinoma and gallbladder cancer, as seen in our patient. Placement of biliary stents can temporarily alleviate symptoms, but surgical excision is the definitive treatment. Our case highlights the importance of including type 1 choledochal cyst in the differential diagnosis even in adult patients with a significantly dilated CBD. A thorough endoscopic and radiological work up should be done to prevent progression to malignancy.Figure 1.: A. CT of the abdomen showing marked dilatation of extrahepatic bile duct with a transition point in the distal intrapancreatic portion. B. Cholangiogram showing Type 1 Choledochal Cyst and Type C Anomalous Pancreaticobiliary Maljunction. C. Spyglass cholangioscopy showing JagwireTM within the CBD (yellow arrow) revealing focal stenotic CBD joining the common channel just above the ventral pancreatic duct (blue arrow). Table 1. - Types of Choledochal Cysts Type Description Type I Fusiform dilation of the CBD Type II Bile duct diverticulum Type III Saccular dilation of the intraduodenal portion of the CBD Type IVa Multiple cysts at intra and extrahepatic ducts Type IVb Multiple cysts at extrahepatic ducts only Type V Fusiform or saccular cystic dilation of the intrahepatic bile duct