It has been suggested that immunological mechanisms involving lymphocyte-mediated damage are important in the characteristic bile-duct damage that occurs in primary biliary cirrhosis and primary sclerosing cholangitis. Because adhesion is necessary for the interaction of lymphocytes with their target structures, we have studied the expression of intercellular adhesion molecule 1, a ligand for the leukocyte adhesion receptor lymphocyte function-associated antigen 1 in the liver of patients with primary biliary cirrhosis and primary sclerosing cholangitis. Strong expression of intercellular adhesion molecule 1 was seen on interlobular bile ducts and proliferating bile ductules in both conditions. In primary biliary cirrhosis, medium-sized ducts, which are spared by the disease, were negative. Minimal bile-duct staining was seen in conditions in which bile-duct damage is not a major feature, such as nonbiliary cirrhosis and acute liver diseases. In patients with cirrhosis from any cause, strong expression of intercellular adhesion molecule 1 was detected on the periseptal hepatocytes adjacent to new connective tissue. The intensity of immunohistochemical staining was recorded using a semiquantitative visual scoring system that was subsequently validated quantitatively by confocal laser scanning microscopy. The expression/induction of intercellular adhesion molecule 1 on bile ducts may be important in the pathogenesis of bile-duct damage in primary biliary cirrhosis and primary sclerosing cholangitis and is further evidence to support an immune pathogenesis in these two conditions. Furthermore, the induction of intercellular adhesion molecule 1 on hepatocytes may be an important factor in the liver-cell damage and fibrosis that occur during the development of cirrhosis.