Autoimmune pancreatitis (AIP) shows a unique spectrum of histologic features and commonly presents with an abundant IgG4-positive (IgG4+) plasma cell infiltration. However, differentiating AIP from other mass lesions, particularly pancreatic cancer [invasive ductal carcinoma (IDC)] can be clinically challenging. In this study, we evaluated the validity of IgG4 and IgG immunohistochemistry of ampullary and periampullary tissue for the diagnosis of AIP. Our study group consisted of 14 resected AIP cases with appropriate ampullary sections. Superficial ampullary tissue and "shouldering" duodenal mucosa were evaluated for several histologic variables. Immunohistochemistry for IgG4 and IgG was performed. The number of IgG4 and IgG-positive plasma cells was counted and an IgG4+ to IgG+ plasma cells ratio (IgG4/IgG ratio) was evaluated. A control cohort was composed of IDC (n=30) and chronic pancreatitis (CP) (n=29). Although an overlap was present between the groups, the overall inflammation and number of plasma cells in and around the ampulla was significantly increased in AIP compared with CP and IDC. Furthermore, although there was some overlap in the crude number of IgG4+ plasma cells of the ampullary and duodenal tissue between AIP, IDC, and CP, an IgG4/IgG ratio, especially of the ampulla, seems diagnostically useful in differentiating AIP from other "mass forming" lesions. When a cut-off of 0.10 was applied, the diagnostic sensitivity and specificity of the ampullary IgG4/IgG ratio was 86% and 95%, respectively. In conclusion, evaluation of ampullary histology and IgG4/IgG ratio might be proven beneficial in discriminating AIP from other mass forming pancreatic lesions.