Introduction: Rare complications of gallbladder disease include fistula formation, which can lead to further complications including pneumobilia, recurrent cholangitis, Mirizzi's, Bouveret's and gallstone ileus. Due to their rarity and the variability in which it manifests, a high degree of suspicion must be present at the start to reduce morbidity and mortality. We present a case of a cholecystoduodneal fistula that presented with massive upper GI bleeding. Case Report: A 75-year-old female patient with a history of myelodysplastic syndrome, coronary artery disease, GERD, urinary incontinence, and hypertension presented to the ED with weakness and vomiting for 1 day, with fevers and altered mental status. In the ED, the patient's vitals were temperature of 104 F, BP 94/52, and HR 102. Initial labs were significant for a white blood cell count of 24,700 and a Hb of 7.1 g/dL. In the ED, the patient experienced an episode of 200 mL of hematemesis and bright red blood per rectum. The patient was hypotensive to 80/40 with positive orthostatics. Intravenous fluids and 5 units of pRBC were given and her blood pressure improved to 115/59. An urgent EGD was performed and showed old blood in the fundus with no source for the bleeding. On day two, the patient had a recurrent episode of massive hematemesis. A repeat EGD showed bubbles emanating from the papilla, with normal gastric mucosa. A CT and MRCP were ordered showing pneumobilia with air extending to the duodenal bulb and the presence of multiple gallstones. The findings were compatible with persistent cholecystoduodenal fistula and a decompressed gallbladder. Blood cultures confirmed gram negative sepsis. The patient was treated with IV vancomycin and zosyn, improved clinically and discharged in stable condition with no more recurrent bleeding episodes. Despite recommendations, surgery was not performed. One year later, the patient returned to the ED with a several day history of weakness, decreased appetite, and weight loss. The patient's labs were significant for a white blood cell count of 15,630, an AST of 225 IU/L, ALT of 207 IU/L, and alkaline phosphatase of 697 IU/L. CT scan and MRI of abdomen and pelvis revealed an approximately 4.7-cm mass with biliary obstruction at the level of the porta hepatis, with obliteration of the previous choledocystoduodenal fistula. Biopsies confirmed gallbladder carcinoma, and a subsequent ERCP was performed with successful palliative biliary stenting. Discussion: In cases of unexplained massive upper GI Bleeding, careful inspection of the papilla may help rule out hemobilia. In this case, the diagnosis of hemobilia helped lead to the immediate treatment and management of a complicated cholecystoduodenal fistula. Disclosure - Dr. David Lee: CSL Behring: consultant: speaker bureau for a symposium.