Antigen presenting dendritic cells (DCs) can serve as sites for HIV replication and as vehicles for transmission of the virus to T cells. It is known that the numbers of DCs in blood is reduced during HIV-1 infection. Here we monitored the two major subsets of blood DCs in 12 individuals undergoing a change, primarily initiation, of highly active antiretroviral therapy. The numbers of plasmacytoid DCs were reliably higher on therapy, although in the 1–3 month interval we followed, these numbers did not return to those seen in HIV uninfected controls. An increase in plasmacytoid DCs was accompanied by an increase in IFN-α production in response to a standard challenge in culture with UV-inactivated herpes simplex virus. The levels of myeloid DCs also demonstrated an increase while on HAART, and these numbers become comparable to the HIV uninfected controls. The numbers of plasmacytoid and myeloid DCs varied inversely with the levels of plasma HIV viremia. These longitudinal studies extend prior work showing that virus infection with HIV leads to a decrease in the number of dendritic cells in blood, and that this can be reversed at least in part by therapy.