Antibodies directed against CD20 (L26, Leu 16, and Bl) are frequently used to determine the presence of B lymphocytes. However, recent publications describe the unexpected presence of CD20-positive T cells in the peripheral blood of normal subjects and occasional T-cell neoplasms that express CD20. To determine the presence of CD20-positive T cells in bone marrow, flow cytometric analysis was performed on 34 aspirate specimens (14 normal, 5 acute lymphoblastic lymphoma [ALL], 5 acute myelogenous leukemia [AML], 4 HIV positive, 2 myelodysplastic/myeloproliferative, 2 chronic myelogenous leukemia [CML], 1 chronic lymphocytic lymphoma [CLL], 1 multiple myeloma). A small population of cells coexpressing CD3 (Leu 4) and CD20dim (Leu 16) was identified in 94% of the specimens, representing 0% to 11% (mean 1.77%) of marrow mononuclear cells and 0% to 22.2% (mean 6.54%) of marrow lymphoid cells. There was no correlation between the percentage of CD20-positive T cells and the CD4:CD8 ratio, patient age, gender, or diagnosis. CD20dim positive cells included immature B cells and CD20-positive T cells. Although evaluation of CD20 expression is useful in delineating B-cell processes, caution should be exercised in interpreting its expression on bone marrow T-lymphoid cells. CD20 expression on T cells may be seen in either normal, reactive, or neoplastic processes.