Flowcytometer has been flourishing since the passed decade in our nation. Many general hospitals use it for routine tests. One of the main usage is immunophenotyping for hematological malignancies. Flowcytometric immunophenotyping (FCM-IM) facilitates accurate diagnosis in both leukemias and lymphoid proliferative disorders (LPDs). It becomes an indispensable criteria in the diagnosis of hematological malignancies. It may also offer additional therapeutically relevant information. Typical patterns of immunophenotype in leukemias and certain types of LPDs are illustrated in figure 1 and figure 2. But variation or overlapping appears often. In order to reach a definitive diagnosis and to have a common interpretation between laboratory to laboratory the following items need not to be ignored: 1. Direct staining and multi-color or multi-parameter flow cytometric analysis is necessary. 2. The phenotype of abnormal cells should be determined on as pure a population as possible therefore CD45/SSC gating is critical for isolating the blast population (abnormal cells). CD45/SSC gating is much better than FSC/SSC gating especially when percentage of blast is low in the sample. Else, each subtype of leukemia has its special CD45/SSC pattern. 3. The phenotype of this certain population should be described clearly but it is not necessary to count the percentage of each marker. 4. > 20% for positive threshold used in indirect staining method may be confused in multi-color direct staining method. 5. Hybrid leukemia diagnosed by FCM only should be in caution since the divergence of antigen/antibody, the lineage infidelity of antigen expression and the biological characteristics of the leukemic cells. Flowcytometer is not a cell counter. "It provides a pattern of information that must be interpreted by a knowledgeable professional in the context of the clinical situation".