T-cell receptor (TCR) complementarily determining region 3 (CDR3) spetratyping analysis was employed to assess the ability of an AIDS virus to disrupt CD4 + T-cell repertoires during the primary infection. Rhesus and pig-tailed macaques infected with simian immunodeficiency virus (SIV)mac 251 and SIVsmmFGb, respectively, were evaluated. Following SIV infection, the macaques exhibited an apparent decline of CD4 + peripheral blood lymphocyte (PBL) counts, which was associated with a change in CDR3 profiles from multiple-length distribution to one- or two-length dominance in the selected TCR Vbeta-expressing CD4 + PBL subpopulations. Molecular analysis of the perturbed cell subpopulations suggested that the CD4 + T cells bearing the dominant CDR3 length were clonally expanded. These results indicate that SIV infection can induce a disruption of macaque CD4 + T-cell repertoires during the primary infection. The finding in this study, therefore, suggests that the virus-induced clonal dominance can contribute to the disruption of CD4 + T-cell repertoires.