Autoimmune pancreatitis (AIP) is defined histologically by periductal and interacinar lymphocytic infiltration. Immunohistochemically, the majority of these lymphocytes are identified as T cells. Epithelial HLA-DR antigen expression was also described as a marker of autoimmunity in this type of chronic pancreatitis. We report 2 cases, a 56-year-old man and a 29-year-old woman, with AIP associated with immune-mediated inflammation of the main duodenal papilla (MDP). Serologically, antinuclear antibody positivity was detected in the male patient. The female patient, treated medically for ulcerative proctitis, had no serological evidence of autoimmune disease. Macroscopic papillitis was present only in the male patient, and endoscopic biopsy samples were taken from this swollen MDP. Since we could not exclude malignancy, a pancreatic head resection was performed in both patients. The histologic and immunohistochemical studies of the resected specimens showed periductal T-lymphocytic infiltration in the pancreatic and papillary tissues. Furthermore, HLA-DR-antigen expression was also demonstrated in epithelial cells of the pancreas and MDP. The immunohistological features of endoscopic biopsy samples from the swollen MDP were identical as in the surgically resected specimens. Immune-mediated inflammation of the MDP may be associated with AIP.