In the French-American-British (FAB) classification of acute leukemia, immunophenotyping is useful to make a diagnosis of megakaryoblastic type (M7) and minimally differentiated acute myeloid leukemia (AML) (M0). However, subtyping of acute lymphocytic leukemia (ALL) is decided with morphological findings, not with immunophenotyping. Furthermore, recently, immunophenotyping revealed the existence of acute leukemia with ambiguous lineage which has both myeloid and lymphoid characters. But the FAB classification does not include this category in it. On the other hand, WHO classification, which was proposed in 1999, is a novel integrated subtyping system which uses all available information, i. e., clinical history, morphology, cytochemistry, cytogenetics, molecular biology, and immunophenotyping. In subclassification of AML, immunophenotyping is useful in the following three situations; 1) distinguishing AML from ALL, 2) subclassification within AML not otherwise categorized, and 3) diagnosis of acute leukemia with ambiguous lineage. Furthermore, for subtyping of ALL, a panel of various lymphocyte markers is used for diagnosis of B- and T-lineage ALL according to lineage and differentiation status. In cases of AML with recurrent genetic abnormalities and AML with multilineage dysplasia, there is no specific immunophenotype by which we can recognize these disease entities. There were four cases of acute leukemia with ambiguous lineage in our series. Three of four cases had Philadelphia chromosome. Many subtypes of acute leukemia in WHO classifiacation have some specific immunophenotypical findings partly. Since this novel classification system is integrated many diagnostic factors, there are some undisposed part and a diversity in a certain subtype possibly. Thus, in the present clinical uses, it might be important to apply WHO classification together with the FAB classification. Furthermore, large scale and prospective clinical investigations are essential to reveal a number of clinical significance of each subtype. In such condition, immunophenotyping might be powerful information in the primary diagnostic step. In the near future, integration of novel molecular and cellular knowledge will enable us to rearrange the classification system and to establish new disease entities more precisely. In such process, there might emerge a number of novel and specific biological markers other than conventional differentiation markers. With these markers, immunophenotypig of acute leukemia might gain further significances in clinical and biological aspects.
