Fas antigen is a receptor that crosslinks with a ligand or antibody initiating a signal transduction cascade that leads to apoptosis. During normal hematopoiesis, Fas antigen is not expressed on CD34+ cells, including premature hematopoietic progenitor cells. Functioning Fas antigen expression is induced by several hematopoietic regulators. These changes may appear not only in the process of differentiation of hematopoietic progenitor cells, but also as a negative feedback mechanism that controls chaotic proliferation of these cells. These findings suggest that the Fas/Fas ligand system is closely related to the maintenance of homeostasis during the process of normal hematopoiesis. Furthermore, increased Fas antigen expression is observed on CD34+ cells from patients with aplastic anemia, suggesting that it might cause bone marrow suppression. The use of Fas-mediated apoptosis of malignant cells as a tool for eliminating hematologic malignancies is promising. Increased Fas ligand expression is observed on natural killer lymphoma cells and may be associated with the pathogenesis of failure of several organs. The Fas/Fas ligand system plays an important role in the physiologic and pathologic processes of hematopoiesis. The development of treatments using this system are forthcoming.