Little is known about the factors that govern the level of HIV-1 replication in infected individuals. Recent studies (using potent antiviral drugs) of the kinetics of HIV-1 replication in vivo have demonstrated that steady-state levels of viremia are sustained by continuous rounds of de novo infection and the associated rapid turnover of CD4+ T lymphocytes. However, no information is available concerning the biologic variables that determine the size ofthe pool ofT cells that are susceptible to virus infection or the amount of virus produced from infected cells. Furthermore, it is not known whether all CD4+ T lymphocytes are equally susceptible to HIV-1 infection at a given time or whether the infection is focused on cells of a particular state ofactivation or antigenic specificity. Although HIV-1 replication in culture is known to be greatly facilitated by T cell activation, the ability of specific antigenic stimulation to augment HIV-1 replication in vivo has not been studied. We sought to determine whether vaccination ofHIV-1-infected adults leads to activation ofvirus replication and the targeting ofvaccine antigen-responsive T cells for virus infection and destruction. Should T cell activation resulting from exposure to environmental antigens prove to be an important determinant of the steadystate levels of HIV-1 replication in vivo and lead to the preferential loss ofspecific populations of CD4+ T lymphocytes, it would have significant implications for our understanding of and therapeutic strategies for HIV-1 disease. To begin to address these issues, HIV-1-infected individuals and uninfected controls were studied by measurement of immune responses to influenza antigens and quantitation of virion-associated plasma HIV-1 RNA levels at baseline and at intervals after immunization with the trivalent influenza vaccine. Influenza vaccination resulted in readily demonstrable but transient increases in plasma HIV-1 RNA levels, indicative of activation ofviral replication, in HIV-1-infected individuals with preserved ability to immunologically respond to vaccine antigens. Activation ofHIV-1 replication by vaccination was more often seen and ofgreater magnitude in individuals who displayed a T cell proliferative response to vaccine antigens at baseline and in those who mounted a significant serologic response after vaccination. The fold increase in viremia, as well as the rates ofincrease of HIV-1 in plasma after vaccination and rates of viral decline after peak viremia, were higher in individuals with higher CD4+ T cell counts. These data indicate that important aspects of the host-virus relationships underlying HIV-1 infection may be gleaned from the careful analysis of interventions that perturb, either positively or negatively, the steady-state equilibrium of virus replication in vivo. The potential adverse consequences of vaccine-induced activation of HIV-1 replication deserve consideration in formulating guidelines for immunization of HIV-1-infected individuals. Given the demonstrated ability of antigenic challenge to activate virus replication in infected individuals, the contribution of immune stimulation to T cell depletion and disease progression in HIV-1-infected individuals are important topics for future study.