Despite considerable evidence that cell activation enhances human immunodeficiency virus-type 1 (HIV-1) replication in vitro, there is very little data on the role of immune activation on in vivo HIV-1 replication. In this study, we examined the effect of influenza vaccination on HIV-1 replication in the peripheral blood of 20 study subjects, and in 14 control subjects who did not receive influenza vaccination. Blood was obtained from each subject on three occasions during the month before vaccination and again on three occasions during the following month. Over the study period, there was little change in levels of proviral DNA in peripheral blood mononuclear cells (PBMCs). However, peak PBMC viral RNA levels after influenza vaccination were significantly increased over the mean of prevaccination values. This change was not observed to the same extent in unvaccinated controls. Therefore, this is the first report showing that HIV-1 replication can increase in temporal association with influenza vaccination. Our results suggest that continued immunologic (antigenic) stimulation may result in increased virus load in vivo. To address the appropriateness of influenza vaccination in HIV-infected patients, expanded studies will be required to examine specific and generalized immune responses to vaccination, and differences in patient response based on disease stage.