The affect of antiretroviral therapy (ART) on HIV-1 recovery from blood monocytes was determined in purified peripheral blood monocyte-derived macrophage (MDM) cultures from HIV-1-infected subjects with undetectable plasma viremia or active viral replication. Additionally, the association between replication-competent HIV-1-infected MDM and neurocognitive status was examined. Fifty-two individual with previous AIDS-defining illnesses receiving nucleoside analogues with and without protease inhibitors or no ART were followed for up to 1.5 years. Detection of plasma viremia significantly correlated with the occurrence of infected monocytes. Viral replication was detected in less than 10% of the MDM cultures from 23 individuals receiving effective antiretroviral therapy. In contrast, approximately 50% of the MDM cultures from 29 individuals with active viral replication and evidence of decreased immune function, including all individuals with neurocognitive impairment, produced detectable virus indicating that a lack of adequate ART results in increased abundance of replication-competent blood monocytes. Proviral DNA levels were a minimum of 13-fold higher in MDM from subjects with active viral replication. The infrequent detection of viral DNA in cultures from individuals receiving effective ART suggested low levels of circulating monocytes harboring replication-incompetent virus. These studies demonstrate that HIV-infected individuals on ART with breakthrough viremia have significantly higher levels of circulating infected monocytes, the precursors of tissue macrophages.