Waldenström macroglobulinemia (WM) is currently defined as lymphoplasmacytic lymphoma involving bone marrow (BM) associated with a serum IgM paraprotein. WM is typically composed of small lymphocytes, plasmacytoid lymphocytes, and plasma cells in variable proportions, which can change after therapy. In this study, we assessed 41 WM cases that required chemotherapy, 39 showing persistent disease in restaging BM specimens. In 10 cases, there was persistent monotypic plasmacytosis in BM in the absence of demonstrable monotypic B cells. The monotypic plasma cells represented 0.5% to 46% of the cellularity and persisted 1 to 50 months after the last course of chemotherapy. The plasma cells were best quantified by immunohistochemical analysis on paraffin sections. We conclude that WM can persist as a pure plasma cell population after therapy. This finding has implications for the immunophenotypic assessment of WM after therapy and may explain persistent IgM paraproteinemia in patients with WM with no evidence of a clonal B-lymphocyte population.