Multiple myeloma is responsible for approximately eight percent of all canine hematopoietic tumors and is accountable for 3.6 percent of all neoplasias affecting the bones of dogs. It is a neoplasia of the plasma cells in the bone marrow. Classically, diagnosis of canine multiple myeloma is made based on the presence at least two of the following four criteria: 1) bone marrow plasmacytosis of greater than five percent, 2) radiographic evidence of osteolysis, 3) monoclonal gammopathy on serum or urine electrophoresis, and 4) Bence-jones proteinuria. Many of the clinical signs and complications associated with multiple myeloma are caused by the overproduction of the M component (immunoglobulins) by neoplastic plasma cells. In dogs, the immunoglobulins produced excessively are IgG, IgA, and IgM (macroglobulinemia). If there is only abnormal over-production of the immunoglobulin light chains, Bence-jones proteinuria is observed. In rare cases, such as a nonsecreting multiple myeloma, some of the criteria for diagnosis may be absent. Diagnosis is then made by the extent of bone marrow plasmacytosis and the presence of neoplastic plasma cells.