Multiple myeloma was diagnosed in two cats with monoclonal hyperglobulinemia, proteinuria, and plasma cell proliferations in bone marrow. An immunoglobulin G-producing myeloma occurred in the vertebral bone marrow of one cat, and twice responded to surgical reduction followed by a combination of local irradiation and chemotherapy. The cat's survival time was approximately 2 years. The other myeloma in a cat that presented with hypercalcemia and renal insufficiency involved visceral organs and produced a biclonal peak due to immunoglobulin A dimer formation on serum electrophoresis. This cat's tumor did not respond to chemotherapy.