Apoptosis is a key for CD4⁺ T cell destruction in HIV-1–infected patients. In this study, human peripheral blood lymphocyte (PBL)-transplanted nonobese diabetic (NOD)-severe combined immunodeficient (SCID) (hu-PBL-NOD-SCID) mice were used to examine in vivo apoptosis after HIV-1 infection. As the hu-PBL-NOD-SCID mouse model allowed us to see extensive infection with HIV-1 and to analyze apoptosis in human cells in combination with immunohistological methods, we were able to quantify the number of apoptotic cells with HIV-1 infection. As demonstrated by terminal deoxynucleotidyl transferase–mediated dUTP nick-end labeling (TUNEL), massive apoptosis was predominantly observed in virus-uninfected CD4⁺ T cells in the spleens of HIV-1–infected mice. A combination of TUNEL and immunostaining for death-inducing tumor necrosis factor (TNF) family molecules indicated that the apoptotic cells were frequently found in conjugation with TNF-related apoptosis-inducing ligand (TRAIL)-expressing CD3⁺CD4⁺ human T cells. Administration of a neutralizing anti-TRAIL mAb in HIV-1–infected mice markedly inhibited the development of CD4⁺ T cell apoptosis. These results suggest that a large number of HIV-1–uninfected CD4⁺ T cells undergo TRAIL-mediated apoptosis in HIV-infected lymphoid organs.