Forty-one patients with chronic fatigue syndrome (CFS), 76 healthy controls matched with the patient group for age range, sex, race, and socioeconomic class, and 22 symptomatic patients with seasonal affective disorder (SAD) had serum sampled for antibodies against 2 Epstein-Barr virus (EBV) replicating enzymes. Abnormal titers of antibodies were found twice as often in CFS patients as controls (34.1% vs. 17.1%), with SAD patients having an intermediate frequency (27.3%). Stratifying for disease severity sharpened the differences considerably, with the sicker CFS and SAD patients having 52% and 50% abnormal tests, respectively; more mildly afflicted CFS and SAD patients had a frequency of abnormal tests in the normal range. Antibodies to EBV DNA polymerase (DNAP) were the more sensitive of the two tests in that they were positive in all cases but one. These findings suggest that antibodies against EBV DNAP may be a useful marker in delineating a subset of patients with severe fatiguing illness for appropriate treatment trials and for monitoring their outcomes.