This volume presents the proceedings of the Falk Symposium No. 127 on Autoimmune Diseases in Paediatric Gastroenterology (IV International Falk Symposium on Paediatric Gastroenterology), held in Basel, Switzerland, on November 8-9, 2001. The symposium focused on the role of the immune system, both the acquired and the innate systems, in inflammatory bowel disease (IBD) in children and adolescents. The innate system has an important fundamental role in host defence by initiating immune responses against potentially deleterious matter. However, a mutation within the innate system may elicit an immune response against the host: hence, an autoimmune response. Chronic autoimmune hepatitis occurs predominantly in young people, and especially in women. Immunological changes are conspicuous. Tissue antibodies are found in a large number of patients. This is a disease of disordered immunoregulation marked by a deficit in suppressor T cells causing the production of autoantibodies against specific hepatocyte surface antigen. Liver membrane protein is found in the sera of patients with autoimmune chronic acute hepatitis and with primary biliary cirrhosis (PBC).