To evaluate the clinical and virologic course of patients with chronic fatigue who had elevated Epstein-Barr virus (EBV) titers, we prospectively followed up 26 patients with serial cultures for EBV in blood and saliva and serial EBV serologic and clinical and psychiatric evaluations, and we compared these results with those for healthy controls. The frequency of isolating EBV in blood or demonstrating EBV infection by in situ hybridization in blood lymphocytes or in saliva was similar in patients and controls. The prevalence and titers of antibody to human herpesvirus type 6 were also similar in the two populations. Patients with chronic fatigue did demonstrate higher in vitro natural killer activity and lower in vitro interleukin 2 production than controls, and patients had a high frequency of DSM-III depressive illness. Over 50% of patients with chronic fatigue improved over the course of follow-up. Improvement was not associated with any discernible change in titers of EBV proteins. No evidence of ongoing EBV infection with either transforming or nontransforming strains was demonstrated in this population of patients with chronic fatigue. Clinically, most patients gradually improve over time.