Multiple myeloma was diagnosed in 60 dogs. Diagnosis was confirmed in each case by observation of greater than 5% plasma cells on examination of a bone marrow aspirate and detection of monoclonal gammopathy of immunoglobulin (Ig) A or IgG. Treatment with melphalan, cyclophosphamide, and prednisone was associated with long-term survival (median, 540 days). Response to therapy was significantly related to prognosis (P less than 0.01), whereas hypercalcemia and Ig light chain proteinuria (Bence Jones) were associated with shorter median survival times.