BACKGROUND: Fas is a member of the tumor necrosis factor (TNF)/nerve growth factor (NGF) receptor family. Triggering of the Fas receptor pathway by its ligand results in apoptosis. Soluble Fas consists of the extracellular region of Fas receptor and it binds to Fas ligand to inhibit the Fas and Fas ligand induced apoptosis. Recently some evidence indicates that the Fas/Fas ligand system represents an important pathway responsible for the induction of apoptosis in bone marrow CD34+ cells of patients with aplastic anemia. METHODS: We measured serum soluble Fas levels in 27 patients with aplastic anemia at diagnosis using ELISA to define the status of soluble Fas in this disorder. RESULTS: Levels of serum soluble Fas in patients with aplastic anemia were lower com-pared with that of normal healthy controls. No difference was noted in the serum soluble Fas levels according to severity of disease. No correlation was found between serum soluble Fas levels and hematologic parameters at diagnosis such as neutrophil count, lymphocyte count, platelet count and corrected reticulocyte count. CONCLUSION: These results indicate that serum soluble Fas levels are decreased in patients with aplastic anemia. Further studies recruiting more patients and measuring Fas receptor on peripheral blood lymphocyte subsets and bone marrow CD34+ cells concomitantly may be helpful to determine pathophysiology of bone marrow failure.