The relationship between T cell activation and human immunodeficiency virus type 1 (HIV-1) replication was studied in HIV-infected subjects, 20 with and 10 without anti-HIV treatment. Expression of Ki-67 proliferation-associated antigen was increased in CD4⁺ and CD8⁺ T cells and correlated with HLA-DR. In subjects without anti-HIV treatment, the plasma HIV-1 RNA level correlated with HLA-DR in CD4⁺ T cells, with Ki-67 in CD8⁺ T cells, and with expression of CD38 in both T cell subsets. A proportion of treated subjects had increased T cell activation despite 4 months of highly active antiretroviral treatment (HAART). In subjects receiving HAART, a high percentage of HLA-DR⁺ CD4⁺ T cells was associated with signs of opportunistic infections. This work supports the concept that, in the natural course of HIV-1 infection, HIV replication itself leads to general T cell activation and that opportunistic infections generate additional CD4⁺ T cell activation and HIV replication.