Infection by HIV is associated with the expansion of monocytes expressing CD16 antigens, but the significance of this in HIV pathogenesis is largely unknown. In rhesus macaques, at least three subpopulations of blood monocytes were identified based on their expression of CD14 and CD16: CD14(high)CD16(-), CD14(high)CD16(low), and CD14(low)CD16(high). The phenotypes and functions of these subpopulations, including CD16(+) monocytes, were investigated in normal, uninfected rhesus macaques and macaques that were infected with SIV or chimeric SHIV. To assess whether these different monocyte subpopulations expand or contract in AIDS pathogenesis, we conducted a cross-sectional study of 54 SIV- or SHIV-infected macaques and 48 uninfected controls. The absolute numbers of monocyte populations were examined in acutely infected animals, chronically infected animals with no detectable plasma virus RNA, chronically infected animals with detectable plasma virus RNA, and animals that died with AIDS. The absolute numbers of CD14(high)CD16(low) and CD14(low)CD16(high) monocytes were elevated significantly in acutely infected animals and chronically infected animals with detectable plasma virus RNA compared with uninfected controls. Moreover, a significant, positive correlation was evident between the number of CD14(high)CD16(low) or CD14(low)CD16(high) monocytes and plasma viral load in the infected cohort. These data show the dynamic changes of blood monocytes, most notably, CD14(high)CD16(low) monocytes during lentiviral infection, which are specific to disease stage.