Chronic fatigue syndrome (CFS) is a debilitating heterogeneous disorder lacking consistent, objective physical or laboratory abnormalities. Among the hypothetical etiologies for CFS are chronic viral infections. The present controlled seroprevalence study found that, among typical CFS patients, evidence of hepatitis C virus (HCV) infection is uncommon. Only one of 36 patients and none of 14 controls were anti-HCV positive. The positive patient had persistent aminotransferase elevations and prior posttransfusion hepatitis. Thus HCV infection is not a common feature of CFS and should not be routinely sought.