In this study, we show that high serum levels of soluble human leukocyte antigens (HLA) class I molecules (sHLA-I, range: 0.7-1.7 μg/ml) and soluble Fas ligand (FasL, range: 0.4-1.9 ng/ml) are detected in patients with acute myeloid leukemia (AML) at diagnosis, compared with healthy donors (HD) (sHLA-I, range: 0.1-0.6 μg/ml; sFasL, range: 0.1-0.4 ng/ml). Patients' sera were able to induce transcription and secretion of FasL in CD8+ T cells, followed by apoptosis in vitro; this apoptosis was inhibited by anti-HLA-I-specific monoclonal antibodies, suggesting that sHLA-I is responsible for cell death. These findings closely relate to the in vivo upregulation of FasL transcription observed in peripheral blood (PB) lymphocytes from AML patients; in the same cells, mRNA for the antiapoptotic proteins Bcl-2 and Bcl-xL was downregulated. Interestingly, caspase-8 and caspase-3, both downstream mediators of death receptor-induced apoptosis, were activated in CD8+ cells of AML patients; one-third of these cells were already apoptotic in vivo, at variance with lymphocytes of HD. These data strongly suggest that in AML, increased levels of sHLA-I molecules may contribute to the elimination of potentially anti-tumor effector cells through a FasL/Fas interaction.