During differentiation-induction therapy of acute promyelocytic leukemia (APL) patients with all-trans retinoic acid (ATRA), a variety of APL-derived bizarre granulocytic cells appear in the peripheral blood. To evaluate the differentiation induction of leukemic cells, we have developed a new scattergram analyzing program with an automated hematology analyzer and compared the data with the flow cytometry measuring the expression of differentiation-associated cell surface antigens, CD11b and CD16. We used the fluorescence intensity and side scatter as parameters of granulocytic maturation in the analysis with the automated hematology analyzer. The analysis of 2 ATRA-treated APL patients and in vitro study using HL-60 cells demonstrated that the levels of fluorescence intensity and side scatter decreased as accompanied with granulocytic maturation, and these changes were parallel with the results of flow cytometry. Our automated scattergram analysis of cell differentiation will contribute to general, objective, and real-time evaluation of differentiation-induction therapy of APL with ATRA.