Flow cytometric analysis is essential to make a diagnosis of hematological malignancy due to the recent development of equipment and establishment of many monoclonal antibodies with high specificity. Under these conditions, we should keep in mind the following factors to provide accurate results to clinicians. First, we should know the factors that affect the error range, which include the method of preserving the sample, dilution of the primary antibody, incubation time and temperature, adjustment of the equipment, and the gating procedure. Second, the integration of individual results is also important. We should utilize all the information, such as the combination of antibodies, and cell features according to the gating and scattered signals. Third, the cellular composition of the sample is also helpful to interpret the results.