Because of recent advances in instrumentation, immunological reagents, and methods for measuring a variety of cell constituents and functions, there has been increasing interest in clinical applications of flow cytometry. Several commercially available flow cytometers and a growing list of monoclonal antibodies make it feasible for clinical laboratories to apply this technology for phenotyping leukocytes in leukemias, lymphomas, immunological disorders, transplantation, and infectious diseases. The ability to measure cell constituents such as DNA, RNA, proteins, enzymes, and hormone receptors has added to the list of applications. Use in conjunction with simultaneous phenotyping with monoclonal antibodies makes such measurements even more valuable. Phenotyping specimens from a large number of patients with lymphomas and leukemias has demonstrated the value of flow cytometry in diagnosis and patient management.