To identify the role of T cells in chronic active Epstein-Barr virus (EBV) infection, EBV and cytokine gene expression was quantified by use of real-time polymerase chain reaction (PCR) among 6 patients who fulfilled the diagnostic criteria for chronic active EBV infection. Four of these patients showed clonal expansion of EBV-infected T cells. Quantitative PCR for EBV DNA in peripheral blood of patients with symptomatic chronic active EBV infection showed higher copy numbers of virus (mean, 1.45 x 10(5) copies/mL) than were seen in blood from patients with infectious mononucleosis (3.08 x 10(3) copies/mL) or with EBV-associated hemophagocytosis (2.95 x 10(4) copies/mL). Fractionated CD3(+) HLA-DR(+) cells from patients with chronic active EBV infection contained higher copy numbers than did CD3(+) HLA-DR(-) cells. Quantitative PCR for cytokines revealed that interferon-gamma, interleukin (IL)-2, IL-10, and transforming growth factor-beta genes were expressed at higher levels in HLA-DR(+) than in HLA-DR(-) T cells. These results suggest that activated T cells in chronic active EBV infection expressed high levels of EBV DNA and both Th1 and Th2 cytokines. EBV-infected T cells may contribute to the unbalanced cytokine profiles of chronic mononucleosis.