To clarify the role of Epstein-Barr virus (EBV) infection and the value of EBV antibody testing in evaluating patients with chronic fatigue, we studied 200 consecutive patients with chronic fatigue (mean duration, 9 years). Complete EBV serologic panels were obtained for 154 patients, 35 (23%) of whom met serologic or clinical criteria for chronic or reactivated EBV infection. We compared these patients with chronic EBV infection (CEBV cases) to 35 age- and sex-matched patients who were selected from the same cohort of fatigued patients but who did not meet the criteria (CEBV control subjects). We found few differences between groups in demographic characteristics, clinical features, and symptoms; CEBV cases were more likely to meet criteria for the proposed chronic fatigue syndrome (14% vs 0%), and to report that they suffered from an influenza-like illness at the onset of their fatigue syndrome (34% vs 12%), that they lost their job because of their fatigue (37% vs 11%), and that their fatigue was improved by recreational activity (26% vs 3%). Physical examination and laboratory testing showed few abnormalities in either group. Psychiatric morbidity was common in both groups, including mood disorders (63% of CEBV cases vs 54% of CEBV controls), anxiety (11% vs 9%) and somatization disorder (9% in each group). We conclude that EBV serologic patterns have little clinical usefulness in evaluating patients with chronic fatigue.