Active T cell suicide (apoptosis) is supposed to be involved in the CD4 § T cell depletion in the course of HIV infection. We investigated the expression of the apoptosis-related antigen Fas on CD4 + T cells from 25 HIVpositive individuals (CDC I-III) and 8 HIV-negative controis by two-colour flowcytometry. In addition, we evaluated: total CD4 count, HIV p24 antigen concentration in serum after immune complex dissociation, and clinical course of infection in HIV-positive individuals. We found a significant increase in mean Fas expression on CD4 + T cells from HIV-positive individuals compared to HIVnegative individuals (85.84+14.92% vs. 64.28+7.59%, P<0.001). Within the HIV-positive group the increase in Fas expression was correlated with the decline in CD4 count (r =-0.76, P<0.001), p24 antigen concentration in serum after immune complex dissociation (r=0.67, P<0.001), and CDC stage (r=0.73, P<0.001). The upregulation of Fas antigen on CD4 cells is associated with CD4 depletion and other virological and clinical marker of disease progression in HIV infection. Key words Acquired immunodeficiency syndrome 9 Human immundeficiency virus 9 Apoptosis 9 Fas antigen (CD95) 9 p24 antigen 9 Immune complex dissociation Abbreviations AIDS Acquired immunodeficiency syndrome 9 CDC Center of disease control 9 CTL Cytotoxic T lymphocyte 9 FasL Fas ligand 9 HIV Human immunodeficiency virus 9 MFI Mean fluorescence intensity