The term “plasmacytoid T-zone cells” has been used to describe distinctive cells that occur in clusters in the paracortex of some reactive lymph nodes. Recently, tumorous proliferations of these cells have been described in several patients with myelomonocytic leukemias. Neither the nature of these cells nor their relationship to myeloid leukemia has been conclusively established. We report the case of a 64-year-old woman with chronic myelomonocytic leukemia who developed lymphadenopathy that proved to be due to tumorous accumulation of plasmacytoid T-zone cells in the interfollicular regions of the lymph nodes. She underwent splenectomy because of symptomatic splenomegaly; the resected spleen also contained aggregates of plasmacytoid T-zone cells, in addition to extramedullary hematopoiesis. On treatment with busulphan and prednisone, the lymphadenopathy resolved and did not recur. The patient died 7 years later with blast transformation of her myelomonocytic leukemia and no recurrence of lymphadenopathy. The aggregates of plasmacytoid T-zone cells were architecturally and cytologically distinct from the leukemic infiltrates of myeloid cells in the spleen, and there was no evidence of differentiation of these cells into myeloid or monocytic cells. A panel of monoclonal antibodies on paraffin sections revealed no lineage-specific T- or B-cell markers (UCHL1-, L26-), and the plasmacytoid cells were positive for CD68 (KP1) and L60 (CD43), as well as faintly positive for 4KB5 (CD45RA) and MB1 (CD45R). They did not stain with antibodies to myeloid lineage antigens CD15, lysozyme, or myeloperoxidase. The combination of clinical, morphologic, and immunologic features of plasmacytoid T-zone cells in this case suggests that these cells may be of monocytic lineage but are not direct precursors of mature monocytic or granulocytic cells, and may not be part of the neoplastic clone in patients withmyelomonocytic leukemia.