The etiology of chronic fatigue syndrome (CFS) is unknown. Some patients have high antibody titers to viral capsid antigen (VCA) and early antigen (EA) of Epstein-Barr virus (EBV), suggesting that reactivation of EBV is involved. We investigated virus load (spontaneous transformation) and immunologic regression of EBV-induced transformation in peripheral blood mononuclear cells (PBMCs) from 10 selected patients with CFS who had high antibody titers to VCA and EA. The outcome was compared with that for nine healthy controls and one patient with severe chronic active EBV infection (SCAEBV). There were no significant differences in viral load between patients and healthy controls. Immunologic regression of in vitro-transformed PBMCs was also equally efficient in patients and controls. The SCAEBV-infected patient and two controls, who were all seronegative for EBV, showed impaired regression. In conclusion, we were unable to demonstrate a role for reactivation of EBV in CFS, even in selected patients with high titers of antibody to VCA and EA of EBV.