Autoimmune pancreatitis(AIP)is a rare form of pan-creatitis characterized by prominent lymphocyte inf iltration and pancreatic f ibrosis resulting in organ dysfunc-tion.The pathogenesis and pathology of AIP remain unknown.A 64-year-old Chinese man presented with symptoms and signs of bile duct obstruction diffuse enlargement of the head of pancreas,elevated IgG levels,and negative autoimmune antibody responses.A pylorus-preserving pancreatoduodenectomy was per-formed and a pancreatic tumor was suspected.Howev-er,periductal lymphoplasmacytic inf iltration and f ibrosis were found in the head of pancreas and nearby organs instead of tumor cells.Four months after surgery,the patient was readmitted because of reoccurrence ofsevere jaundice and sustained abdominal distension.Prednisone 30 mg/d was administered orally as an AIP was suspected.One and a half months later,the symp-toms of the patient disappeared,and globulin,amino-transferase and bilirubin levels decreased signif icantly.Over a 9-mo follow-up period,the dose of prednisone was gradually decreased to 10 mg/d and the patient remained in good condition.We further demonstrated dominant CD3+/CD8+ populations,CD20+ cells and a few CD4+ cells in the pancreatic parenchyma,duo-denum and gallbladder wall by immunohistochemical assay.This AIP case presented with signif icant CD8+ T lymphocyte inf iltration in the pancreas and extra-pan-creatic lesions,indicating that this cell population may be more important in mediating AIP pathogenesis than previously known and that AIP might be a poorly defined autoimmune disease with heterogeneous pathogenesis. Autoimmune pancreatitis (AIP) is a rare form of pan-creatitis characterized by prominent lymphocyte infiltration and pancreatic f ibrosis resulting in organ dysfunc tion. Pathogenesis and pathology of AIP remain unknown. A 64-year-old Chinese man presented with symptoms and signs of bile duct obstruction diffuse enlargement of the head of pancreas, elevated IgG levels, and negative autoimmune antibody responses. A pylorus-preserving pancreatoduodenectomy was per-formed and a pancreatic tumor was suspected. Host-er, periductal lymphoplasmacytic infiltration and f ibrosis were found in the head of pancreas and nearby organs instead of tumor cells .Four months after surgery, the patient was read because of recorcurrence ofsevere jaundice and sustained abdominal distension. Prednisone 30 mg / d was administered orally as an AIP was suspected.One and a half months later, the symp-toms of the patient disappeared, and globulin, amino-transferase and bilirubin levels decreased signif icantly. Over a 9-mo follow-up period, the dose of prednisone was gradually decreased to 10 mg / d and the patient remained in good condition. Further CD3 + / CD8 + populations, CD20 + cells and a few CD4 + cells in the pancreatic parenchyma, duo -denum and gallbladder wall by immunohistochemical assay. This AIP case presented with signif icant CD8 + T lymphocyte infiltration in the pancreas and extra-pan-creatic lesions, indicating that this cell population may be more important in mediating AIP pathogenesis than previously known and that AIP might be a poorly defined autoimmune disease with heterogeneous pathogenesis.