Although initially thought to be a B lineage restricted antigen, low "density" or antibody binding capacity (ABC) CD20 has recently been detected on subset(s1 of normal T lymphocytes (Hultin et al.: Cytometry 14:196-204, 1993). We report low ABC CD20 expression in three (two children, one adult) cases of T-acute lymphoblastic leukemia (T-ALL). CD20 and other pertinent antigens were detected using a direct dual color method with a Becton Dickinson FACScan@ flow cytometer and Simulset@ software. Only one cell population based on light scatter was noted in each case that immunophenotypically represented almost a pure population of malignant cells expressing T lymphocyte antigens (for example, CD7 98%, 92%, and 1 OO%, respectively). A total of 95%, 87%, and 79% of the cells from the three cases expressed CD20 with an unusual low ABC compared to the customary "bright" CD20 expression on normal B lymphocytes. Other B lymphocyte associated antigens, such as CD19, CD22, Dr, and immunoglobulin light chains, were negative. Eleven other T lymphocytic malignancies from 1991 to 1993 were CD20 negative, including three other case of T-ALL (one adult and two children). One unusual case of intestinal small lymphocytic non-Hodgkin's lymphoma with a natural killer/T lymphocytic immunophenotype not described in this report appeared to be ~~2 0 " d i m " + . Low ABC CD20 expression by T lymphocytic malignancies may provide a more unique immunophenotypic "fingerprint" to help support the diagnosis of T cell neoplasia vs. normalireactive T cells (for example, low ABC CD20 cells represent only 2.4 f 1.5% of normal peripheral blood lymphocytes). This characteristic might also facilitate monitoring patients for residual or recurrent disease. The prognostic significance of CD20 co-expression is uncertain and must await further studies. o 19% Wiley-Liss, Inc.