Studies published previously by this laboratory have demonstrated that patients with AIDS have widely disseminated, active infections with HHV-6 at the time of their death. However, it remains unclear when in the course of the human immunodeficiency virus (HIV) infection the active HHV-6 infection first appears. To address this question, lymph node biopsies from 10 HIV-infected patients were analyzed for active human herpesvirus 6 (HHV-6) infections by immunohistochemical staining. Eight of the biopsies carried the histologic diagnosis of follicular hyperplasia; the other two were characterized as having follicular involution with histiocytosis and reactive lymphadenitis. In total, 10 of 10 (100%) of the lymph nodes studied contained cells productively infected with HHV-6; in contrast, three lymph nodes with follicular hyperplasia and four normal lymph nodes from patients not infected with HIV were negative for HHV-6 infection. Of special note, the absolute CD4+ lymphocyte counts of 75% (6/8) of the HIV-infected individuals included in these studies were > 200/mm3 at the time of their lymph node biopsy. The A variant of HHV-6 was found to be the predominant form of the virus present in the lymph node biopsies from all of these HIV-infected patients, and in vitro studies demonstrated that exposure of monocytic cells carrying latent HIV to HHV-6A resulted in massive upregulation of HIV replication from latency. Thus, active HHV-6 infections appear relatively early in the course of HIV disease, and in vitro studies suggest that the A variant of HHV-6 is capable of breaking HIV latency, with the potential for helping to catalyze the progression of HIV infections to AIDS.