The specific in vitro disturbance of capacities ascribed to Th1 cells in HIV-infected individuals suggests a switch from Th1 to Th2 lymphokine secretion. Indeed, when T cell clones are generated from HIV-infected individuals compared with controls, an increased percentage of Th0 clones is present upon HIV infection. We studied cytokine production in the supernatant of in vitro activated PBMC from a large group of HIV-infected patients at various stages of infection. IL-2, IFN-gamma, IL-4, IL-5, and IL-10 production all were decreased significantly, which does not support a switch to Th2 lymphokine secretion and is possibly due to the generalized impaired response of T cells from HIV-infected individuals to activation signals in vitro. Therefore, we investigated the capacity of single cells to produce a certain cytokine. Intracellular staining of IL-4- and IFN-gamma-producing cells revealed that T cells from HIV-infected individuals contained decreased numbers of IFN-gamma-producing cells, in the presence of normal percentages of cells with the capacity to produce IL-4. This resulted in significantly decreased IFN-gamma/IL-4 ratios in both CD4+ and CD8+ T cells. Thus, in agreement with previous findings in T cell clones, we conclude, from cytokine production upon stimulation of T cells in vitro, that there is a change in the cytokine balance to the Th2 side in HIV infection due to decreased Th1 and preserved Th2 cytokine production.