Acute gastrointestinal bleeding is one of the most frequent medical emergencies. The most common causes are inflammations, ulcers, neoplasms and angiodysplasias. They can usually be diagnosed and treated via endoscopy, but less frequent causes, which require increased diagnostic and therapeutic efforts, must also be considered. We present the case of a 53-year-old patient with gastrointestinal bleeding. He was diagnosed with transverse colon bleeding in another hospital. A bleeding diverticulum was assumed. Following admission to our hospital, a bleeding through the ampulla of Vater was discovered via gastroscopy. The CT scan showed arterial bleeding into the gallbladder. We immediately performed a laparotomy and found a cholecysto-colic fistula as the cause of the bleeding into the transverse colon.