Acute cholecystitis is inflammation of the gallbladder. It usually occurs when a gallstone blocks the cystic duct. Gallstones are small stones, usually made of cholesterol, that form in the gallbladder. The cystic duct is the main opening of the gallbladder. Gallstones are very common, affecting about 1 in 10 adults in the UK. They don't usually cause symptoms, but they can occasionally cause episodes of pain (biliary colic) or acute cholecystitis. Acute cholecystitis is potentially serious.