We report a case of coexisting renal lymphoma and multiple myeloma. A 77-year-old man was referred to our hospital with a diagnosis of cancer in his right kidney. Biopsies of the renal mass and bone marrow were performed because of hyperproteinemia and biclonal gammopathy. Immunohistochemical staining and Southern blot analysis of the tumor's DNA were diagnostic for renal lymphoma. To our knowledge, this is the first time the coexistence of malignant renal lymphoma and multiple myeloma has been reported.