Multiparametric flow cytometry has become an indispensable but complex tool for the diagnosis of acute leukemias. Interpretation of immunophenotypic data within a six-parameter analytical space relies on the standardization and validation of the instrument, the reagents, and the procedure. To address whether or not residual normal lymphocytes, usually present within leukemic samples, can serve as internal quality control for fluorescence intensity, 116 leukemic and 35 normal samples were analyzed.