Pathogenesis of biliary sludge. The increasing application of ultrasonography in biliary tract disease had led to more frequent recognition of an old disorder--"biliary sludge." Sludge is detected on ultrasound as low-amplitude echoes without acoustic shadowing. It layers in the most dependent part of the gallbladder and shifts with positioning. Particulate matter in bile, such as cholesterol monohydrate crystals, has been shown to be echogenic. Agglomeration of these crystals in biles with high mucus content accounts for the layering and the characteristic appearance of the movement of sludge with alteration in patient position. Within the gallbladder, the stability of the vesicular form of cholesterol and protein-lipid interactions are important determinants of cholesterol precipitation. In mixed and pigment gallstones, the equilibrium between ionized and unionized calcium and the hydrolysis of conjugated bilirubin are also important factors. Although the risk factors contributing to the formation of gallbladder sludge have not been critically examined, it is now known that in some instances sludge can produce biliary pain and can be associated with acalculous cholecystitis, recurrent pancreatitis and, ultimately, the formation of gallstones. A better appreciation of the pathogenesis of sludge formation can help in the understanding of the genesis of gallstones and also perhaps in understanding other documented but poorly understood biliary and pancreatic disorders. 