Biliary sludge is a deposit of tiny stones or crystals made up of cholesterol (a type of fat), calcium bilirubinate, and other calcium salts. Some doctors refer to biliary sludge as microlithiasis, pseudolithiasis, or biliary sand.

The relationship between biliary sludge and gallbladder symptoms is not well understood. Experts debate how important biliary sludge is. Some think it is not important, while others believe that having biliary sludge increases the risk of forming gallstones. The disagreement comes from the fact that most people who have biliary sludge do not develop any symptoms; the sludge usually disappears on its own. Also, only a small number of people who have biliary sludge actually develop gallstones.