Cholangitis is an infection of the bile duct system. In order for cholangitis to be present, there must be an obstruction to bile flow and presence of bacteria within the bile ducts. The most common cause of cholangitis is infection of the bile ducts due to blockage by a gallstone. The infection may occur suddenly (acute) or may be chronic.

Other causes include a backflow of bacteria from the small intestine, a blood infection (bacteriemia), or following a diagnostic procedure such as an endoscopic examination. The infection causes pressure to build up in the bile duct system.