A 68-year-old man presented with a solid mass at the left renal pelvis and ureter with multiple systemic lymphadenopathies and a mass with a cavity in the right lower lobe of the lung. While a transbronchial lung biopsy revealed no malignancy, a biopsy of the renal pelvis showed marginal zone lymphoma with polyclonal IgG4-positive cells. The serum IgG4 level and presence of a bilateral orbital mass suggested Mikulicz disease. The lesions shrank following the administration of steroids. A rebiopsy confirmed lung adenocarcinoma, and its background showed IgG4-positive cells a year later. IgG4-related diseases require careful follow-up because they can be complicated by malignancy.