Cold agglutinin disease (CAD) is a rare autoimmune hemolytic anemia, classified into primary and secondary types. Secondary CAD accompanies infection or malignant disease, most often lymphoma, whereas primary CAD frequently represents a lymphoproliferative bone marrow disorder characterized by clonal expansion of B cells. Here, I describe a case of lymphoplasmacytic lymphoma (LPL) developed 6 years after amelioration of primary CAD by rituximab monotherapy. A 54-year-old Japanese woman was diagnosed with primary CAD characterized by a small fraction of B lymphocytes and kappa laterality in the peripheral blood. M-protein was not detected by immuno-electrophoresis. The patient achieved remission following two courses of rituximab monotherapy. The level of IgM was specifically decreased, although levels of IgG and IgA were slightly increased. Six years after rituximab monotherapy, she developed LPL without CAD recurrence. Flow cytometry performed on bone marrow specimens revealed that lymphoma cells were positive for CD19 and CD20 with kappa laterality. The lymphoma may have transformed from clonal B lymphocytes at presentation of CAD. Rituximab monotherapy induced remission of CAD by specific decrease of IgM level, but did not eliminate the clonal B lymphocytes that may have progressed to LPL. This experience may provide clues toward the understanding of the pathophysiology of primary CAD with clonal lymphoproliferative disease of the bone marrow.