Biliary sludge: a critical update. Biliary sludge has been for many years a poorly defined entity, usually with low amplitude, nonshadowing echoes within the most dependent part of the gallbladder, which shift under the influence of postural changes. From a sonographic point of view, the detection of sludge implies the coexistence of small-sized, solid components and of a gel-like embedding material. The chemical nature of biliary sludge has recently been recognized to be predominantly composed of a coaggregate of cholesterol monohydrate crystals and liquid crystalline droplets, and in some cases, such as obstructive jaundice or symptomatic liver diseases, by bilirubin granules, all embedded in a gel matrix of mucous glycoproteins. From a pathogenic point of view, biliary sludge is often associated with biliary stasis, or with conditions characterized by impaired gallbladder contraction, such as prolonged total parenteral nutrition, fasting, and pregnancy. Other causes include mucus hypersecretion, which may favor cholesterol nucleation and crystal growth, and bile infection. Sludge may be an intermediate step in the formation of different types of stones. From an epidemiological point of view, sludge is quite rare in the asymptomatic, free-living population, but may be common in selected series of symptomatic patients. From a clinical point of view, sludge often has a fluctuating course, including frequent disappearances and reappearances, suggesting that the early stages of gallstone formation are reversible. 