Based on histological and immunohistochemical exami-nation of various organs of patients with autoimmune pancreatitis (AIP), a novel clinicopathological entity of IgG4-related sclerosing disease has been proposed. This is a systemic disease that is characterized by ex-tensive IgG4-positive plasma cells and T-lymphocyte inf iltration of various organs. Clinical manifestations are apparent in the pancreas, bile duct, gallbladder, sali-vary gland, retroperitoneum, kidney, lung, and pros-tate, in which tissue fi brosis with obliterative phlebitis is pathologically induced. AIP is not simply pancreatitis but, in fact, is a pancreatic disease indicative of IgG4-related sclerosing diseases. This disease includes AIP, sclerosing cholangitis, cholecystitis, sialadenitis, retro-peritoneal fi brosis, tubulointerstitial nephritis, intersti-tial pneumonia, prostatitis, inflammatory pseudotumor and lymphadenopathy, all IgG4-related. Most IgG4-related sclerosing diseases have been found to be as-sociated with AIP, but also those without pancreatic involvement have been reported. In some cases, only one or two organs are clinically involved, while in oth-ers, three or four organs are affected. The disease occurs predominantly in older men and responds well to steroid therapy. Serum IgG4 levels and immunos-taining with anti-IgG4 antibody are useful in making the diagnosis. Since malignant tumors are frequently suspected on initial presentation, IgG4-related scleros-ing disease should be considered in the differential diagnosis to avoid unnecessary surgery. Based on histological and immunohistochemical exami-nation of various organs of patients with autoimmune pancreatitis (AIP), a novel clinicopathological entity of IgG4-related sclerosing disease has been proposed. This is an systemic disease that is characterized by ex-tensive IgG4-positive plasma cells and T-lymphocyte infiltration in various organs. Clinical manifestations are apparent in the pancreas, bile duct, gallbladder, sali-vary gland, retroperitoneum, kidney, lung, and pros-tate, in which tissue fi brosis with obliterative phlebitis is pathologically This disease includes AIP, sclerosing cholangitis, cholecystitis, sialadenitis, retro-peritoneal fi brosis, tubulointerstitial nephritis, interstitial pneumonia, prostatitis, inflammatory pseudotumor and lymphadenopathy, all IgG4-related. Most IgG4-related sclerosing diseases have been found to be as-soci ated with AIP, but also those without pancreatic involvement have been reported. In some cases, only one or two organs are clinically involved, while in oth-ers, three or four organs are affected. The disease occurs predominantly in older men and responds well Serum IgG4 levels and immunos-taining with anti-IgG4 antibodies are useful in making the diagnosis. Since malignant tumors are frequently suspected on initial presentation, IgG4-related sclerosing diseases should be considered in the differential diagnosis of avoidance unnecessary surgery.