We analyzed the potential causes of the increased susceptibility to apoptosis of peripheral lymphocytes from a large cohort of HIV-infected persons that we followed during a 3-yr period. By using quantitative cytofluorometric methods, we demonstrate that all lymphocyte populations were contributing proportionally to the apoptotic population in both groups of donors, but the percentage of T and B cells involved in this cell death process was significantly increased in HIV-infected persons. To study the relationship between the increased apoptosis in HIV infection and the activation state of the immune system, we analyzed cell surface expression of activation markers on apoptotic and nonapoptotic cells. We found that in the chronic phase of HIV infection, 50 to 60% of the apoptotic cells exhibited an activated phenotype (they were HLA-DR+, CD38+, CD45RO+, and Fas+), and interestingly, the CD45RO+ subset appeared to be more prone to apoptosis in HIV-positive persons. This study also indicates that the activated phenotype found on apoptotic cells was not a distinctive feature of patients' lymphocytes since it was in similar proportion in apoptotic cells from control lymphocytes. However, a significant correlation was found between the intensity of anti-CD3-induced apoptosis in both CD4 and CD8 T cells from HIV-infected persons and their in vivo expression of CD45RO and HLA-DR molecules. Finally, a significant correlation was found between the intensity of spontaneous or anti-CD3-induced apoptosis in total lymphocytes and disease progression; this was confirmed when the CD4 and CD8 T cell subsets were analyzed separately. Altogether these observations indicate that the increased susceptibility to apoptosis of peripheral T cells from HIV-infected persons correlates with disease progression and support the hypothesis that the chronic activation of the immune system occurring throughout HIV infection is the primary mechanism responsible for this cell deletion process.