A 66-year-old Japanese man with an 11-year history of Mikulicz's disease (MD) received continuous corticosteroid administration. At age 58, a left renal pelvic mass was identified and diagnosed as an IgG4-related inflammatory pseudotumor. The patient underwent an operation to remove the tumor. Subsequently, he contracted repeated pulmonary infections and eventually died of severe gastrointestinal bleeding. Autopsy revealed systemic lymph node swelling and infiltration in some organs, and diffuse large B-cell lymphoma (DLBCL) was diagnosed. These findings suggest that an IgG4-related disease can be causally related to the development of malignant lymphoma through the occurrence of mucosa-associated lymphoid tissue lymphoma.