A 61-year-old Japanese man with IgG4-related autoimmune pancreatitis was referred to our hospital because of perspiration during food intake. Abdominal computed tomography (CT) with contrast media revealed multiple mesenteric lymphadenopathies. An open surgical abdominal biopsy and subsequent histopathological analysis revealed abnormally large lymphoid cells that were negative for CD3, CD5, and c-myc and positive for CD20 and bcl-2, leading to a diagnosis of diffuse large B-cell lymphoma. Here, we discuss the risk of malignancies, particularly malignant lymphoma in patients with IgG4-related disease. The importance of pathological analysis to reach the appropriate diagnosis in such cases should be emphasized.