Autoimmune pancreatitis (AIP) is a peculiar type of pancreatitis of presumed autoimmune etiology. AIP is diagnosed currently based on characteristic radiological findings (irregular narrowing of the main pancreatic duct and enlargement of the pancreas) in combination with serological findings (elevation of serum IgG, and IgG4, and presence of autoantibodies), clinical findings (elderly male preponderance, occasional extrapancreatic lesions, and favorable response to steroid therapy), and histopathological findings (dense infiltration of IgG4-positive plasma cells and T lymphocytes with fibrosis and obliterative phlebitis in the pancreas). Based on histological and immunohistochemical examination of various organs of AIP patients, a novel clinicopathological entity of "IgG4-related sclerosing disease" has been proposed. This is a systemic disease that is characterized by extensive IgG4-positive plasma cell and T lymphocyte infiltration of various organs. Clinical manifestations are apparent in organs such as the pancreas, bile duct, gallbladder, salivary or lacrimal gland, and retroperitoneum, where tissue fibrosis with obliterative phlebitis is pathologically induced. AIP is not simply a form of pancreatitis but rather a pancreatic lesion reflecting an IgG4-related sclerosing disease. This disease includes autoimmune pancreatitis, IgG4-related sclerosing cholangitis, IgG4-related cholecystitis, IgG4-related sialadenitis, IgG4-related retroperitoneal fibrosis. In some cases, only 1 or 2 organs are clinically involved, while in others 3 or 4 organs are affected. Serum IgG4 levels and immunostaining with anti-IgG4 antibody are useful in establishing the diagnosis. Since malignant tumors are frequently suspected on initial presentation, IgG4-related sclerosing disease should be considered in the differential diagnosis to avoid unnecessary surgery.