Immunologic marker studies of the lymphoid leukemias have greatly improved the precision of diagnosis of these disorders by providing specific information regarding the lineage and stage of maturation of the malignant cells. Such studies have also enhanced our understanding of normal lymphocyte development, permitting reproducible identification of lymphoid cells in discrete developmental stages. By elucidating the functions of lymphoid cell differentiation antigens, it has been possible to gain insight into the signal transduction mechanisms by which these cells interact among themselves and with other cell types. Similar studies have shown that ALL is an immunophenotypically heterogeneous disease with clinically important subtypes representing clonal expansions of lymphoblasts at different stages of maturation. Furthermore, successful correlation of immunophenotype with certain karyotypic and molecular abnormalities, which appear to underlie most or all leukemias, were made possible by the inclusion of immunologic marker assessment. Interestingly, many of these phenotype-related abnormalities have involved either the Ig or TCR genes, thus providing additional clues to the mechanisms of leukemogenesis. Knowledge of the immunologic features of leukemic cells has been essential for the generation of phenotype-specific response data in the context of modern therapy for ALL. With wider use of intensive treatment, the traditional prognostic distinctions among immunophenotypes have begun to disappear; however, certain classes of agents have more favorable toxicity/efficacy ratios against some immunophenotypes than others, justifying continued efforts to target therapy by immunologic species of ALL. Antibody-toxin conjugates, or immunotoxins, have induced complete responses in preliminary trials and may prove clinically useful, perhaps in combination with chemotherapy, if their toxic side effects can be controlled. Finally, immunologic markers may serve as sensitive targets for the detection of minimal residual disease; the clinical usefulness of this approach will depend on prospective comparisons with molecular methods.