Dear Editor, Multiple myeloma, a hematologic malignancy characterized by the proliferation of a neoplastic plasma cell clone in the bone marrow, accounts for 10% of hematologic malignancies [1]. The common presenting features, such as anemia, bone pain, hypercalcemia, and increasing susceptibility to infections, are caused by the infiltration of plasma cells and changes in plasma protein [2]. Large amounts of monoclonal protein and a loss of functional antibody production can cause immunological dysfunction and distinct laboratory findings. In multiple myeloma, an ABO discrepancy can result from protein abnormalities causing rouleaux formation or pseudoagglutination in the blood group test [3]. Here, we report a novel case of ABO discrepancy in a patient with IgA kappatype multiple myeloma caused by the loss of isoagglutinin. A 78-year-old woman who had been suffering from multiple myeloma (IgA, kappa type) for 8 years was admitted for a scheduled chemotherapy. The total protein (9.6 g/dl) was elevated due to a large amount of IgA (4,430 mg/dl), whereas, the levels of uninvolved immunoglobulins were reduced (IgG, 236 mg/dl and IgM, <20 mg/dl). In the peripheral blood smear, rouleaux formation was prominent; moreover, her decreased hemoglobin level (4.5 g/ml) necessitated blood transfusion. On blood group testing, the cell type was B+; but anti-A was not detected in her serum. To confirm her blood type, the polymerase chain reaction with restriction fragment length polymorphism was performed and her genotype of ABO*B/O was revealed. She was transfused with five units of B+irradiation-filtered packed red cells without complication before initiating conventional-dose chemotherapy. ABO discrepancies can be caused by the loss of expected anti-A or anti-B isoagglutinins with agammaglobulinemia or hypogammaglobulinemia in primary and secondary immunodeficiencies [4–6]. In multiple myeloma, the levels of the uninvolved immunoglobulin isotypes are usually reduced, but an ABO discrepancy due to the loss of isoagglutinin in multiple myeloma is rarely reported. To the best of our knowledge, only a single study has reported on multiple myeloma with ABO discrepancies due to the loss of isoagglutinins (three cases) [7]. While the monoclonal chains in these cases were the IgG kappa, free lambda, and IgG lambda types, respectively, our patient had IgA kappatype monoclonality. A feature common to all of these cases was the severe loss of uninvolved IgM (Table 1). The isoagglutinins produced by blood group A or B patients consist predominantly of IgM molecules [8]. Therefore, the loss of IgM in these patients would be the reason why the Hwi-Joong Yoon and Tae Sung Park equally contributed to this work as corresponding authors.