What is cholecystitis? Cholecystitis is inflammation of the gallbladder, a small organ near the liver that plays a part in digesting food. Normally, fluid called bile passes out of the gallbladder on its way to the small intestine. If the flow of bile is blocked, it builds up inside the gallbladder, causing swelling, pain, and possible infection. What causes cholecystitis? A gallstone stuck in the cystic duct, a tube that carries bile from the gallbladder, is most often the cause of sudden (acute) cholecystitis. The gallstone blocks fluid from passing out of the gallbladder.