A concept of "autoimmune pancreatitis" has recently been proposed. Computed tomography, magnetic resonance imaging or ultrasonography can demonstrate the diffusely enlarged pancreas with its so called "sausage-like" appearance. Hypergammaglobulinemia, increased serum levels of total IgG or IgG4, positive autoantibodies such as antinuclear antibody, anti-lactoferrin antibody, anti-CA-II antibody and rheumatoid factor have often been observed in patients with autoimmune pancreatitis. Microscopic findings have shown fibrotic changes involving infiltration of lymphocytes and plasmacytes, and often obliterative phlebitis in the pancreas. The major lymphocytes infiltrating the zone around the pancreatic duct were T cells producing IFN-g. HLA-DR was also expressed on the pancreatic duct and acinar cells as were lymphocytes. It is important to make the diagnosis of a diffusely enlarged pancreas based on clinical laboratory findings and pancreatic imaging such as narrowing pancreatogram. Laboratory data, pancreatic images and diabetes mellitus in most patients improved after steroid treatment. In conclusion, autoimmune pancreatitis appears to be a unique clinical entity.