In order to investigate the levels of HHV-6 infection and elevated antibodies to HHV-6 in HIV-1-infected asymptomatic and symptomatic patients, peripheral blood mononuclear cells were (PBMC) cultured. As patients progressed from asymptomatic HIV infection to AIDS, there was a concurrent increase in replicating HHV-6. Plasma obtained from several of these patients showed the presence of IgM antibody and a significantly elevated level of HHV-6 IgG antibody. Serial samples of plasma from 10 AIDS patients collected over a period of 4 years were assayed for the detection of HHV-6 core protein (gp116/64/54) by antigen capture ELISA. The results demonstrated that either a persistent infection or reactivation can occur based on the degree of fluctuation in HHV-6 antigen detected. ELISA to HHV-6 purified viral proteins, i.e., early (p41/38) and late (gp110), demonstrated that IgG antibody to gp110 did not differentiate between HIV-1-infected and healthy donors. IgG and IgM antibody to p41/38, however, showed a significantly higher prevalence in HIV-1-infected individuals (56.7-85.3%) than in normal healthy donors (19.0%), suggesting virus activation. PBMC culture from the AIDS patients expressing significant peaks of HHV-6 core antigen (gp116/64/54) in their plasma showed that in most cases, HHV-6 early and late antigens were detectable; however, those patients with consistently low antigen peaks had no detectable antigens in their PBMC. Only 55% of PBMC cultures established from IgM antibody-positive HIV-1-infected asymptomatic and AIDS patients expressed HHV-6 antigens in the short-term cultures, but HHV-6 antigens could not be demonstrated in PBMC culture from 4 IgM-antibody-positive healthy donors. HHV-6 isolates obtained from the HIV-1-positive patients were predominantly HHV-6 variant A, compared to healthy donors. Based on the data presented here, it is evident that the levels of HHV-6 infection increased in HIV-1-infected asymptomatic individuals as they progressed to AIDS. Our immunovirological data on HHV-6-infected individuals with HIV infection support a role for HHV-6 in the pathogenesis of AIDS. We believe that simultaneous active infection with HIV-1 and HHV-6 may contribute to enhanced immune suppression perhaps leading to disease manifestations.