A case is reported of lymphoplasmacytoid lymphoma (LPL) associated with a monoclonal immunoglobulin (Ig) M and cold agglutinin disease (CAD) that was successfully treated with rituximab. A 52-yr-old male was admitted with a direct antiglobulin test positive haemolytic anaemia and thrombocytopenia associated with monoclonal IgM. Bone marrow examinations disclosed the marked infiltration of medium-sized lymphoma cells with plasmacytoid differentiation that indicated non-Hodgkin's lymphoma of B-cell origin (LPL). Prednisolone and combination chemotherapy were temporarily effective for both anaemia and thrombocytopenia, although these strategies became refractory and bone marrow lymphoplasmacytosis persisted. CAD ameliorated, and the serum level of IgM decreased in association with the disappearance of lymphoma cells and clonal rearrangement of the Ig heavy chains in the bone marrow after treatment with rituximab. Rituximab played a significant role in the treatment of refractory CAD associated with LPL.