Flow cytometry (FCM), which has been elaborated and refined in conjunction with the development of both immunological engineering and information technology, is now indispensable for the diagnosis and treatment of patients with leukemia and lymphoma. FCM had better be regarded as being complementary to, but not competitive with, immunohistochemistry. Histopathologic features are often useful not only for interpretation of the FCM data, but also in evaluating quality of samples used for FCM. In interpretation of the data, one should not put excessive emphasis on bar chart, because conversion of dot plotting to it is associated with loss of many data. Although FCM gives digital data on each thousands of cells, correct interpretation of the data requires an analogue standpoint.