Lymphoplasmacytic lymphoma involving the bone marrow can be difficult to diagnose, and pathological features that predict the presence of associated Waldenström's macroglobulinemia have yet to be identified. To address these issues, marrow histology, immunohistochemistry, and flow cytometry were studied from 35 lymphoplasmacytic lymphoma cases that had comprehensive clinical assessment for Waldenström's macroglobulinemia. In all cases, the plasma cells were analyzed by a novel 6-color flow method. Both immunohistochemistry and flow cytometry were useful in identifying the lymphoid and plasmacytic disease components. In 19 cases, immunohistochemistry revealed an earlier unrecognized pattern of plasma cell infiltration in which they were physically separate from the lymphoid infiltrates. B-cell flow cytometry revealed monotypic cells in 96% of the cases. Approximately half of these were CD5 and/or CD23 positive, although none had features of chronic lymphocytic leukemia, and none of the B cells had flow cytometric features suggesting plasmacytic differentiation. In contrast, highly sensitive 6-color plasma cell flow cytometry revealed monotypic cells in 32 of the 35 cases; in 20 cases, the pattern of CD38 and CD138 coexpression detected was identical to that seen in plasma cell malignancies such as multiple myeloma. In 18 of these 20 lymphoplasmacytic lymphoma cases, these plasma cells were CD19 positive, distinguishing them from those of true plasma cell neoplasms, which are CD19 negative. It is interesting that the two lymphoplasmacytic lymphoma cases with CD19-negative plasma cells had an IgG isotype serum paraprotein. Apart from this, no other pathological correlates of the clinical or laboratory features of symptomatic Waldenström's macroglobulinemia were identified.