Chronic fatigue syndrome (CFS) is a newly-recognized clinical entity characterized by chronic, debilitating fatigue lasting longer than six months. Common associated findings are chronic and recurrent fever, pharyngitis, myalgias, adenopathy, arthralgias, difficulties in cognition and disorders of mood. In the majority of patients, the illness starts suddenly with an acute, 'flu-like' illness. The following abnormalities are seen with some frequency although none are seen in all patients: lymphocytosis, atypical lymphocytosis, monocytosis, elevation of hepatocellular enzymes, low levels of antinuclear antibodies, low levels of immune complexes. Clinical and serologic studies suggest an association of CFS with all of the human herpesviruses, particularly Epstein-Barr virus (EBV) and the recently-discovered human B-lymphotropic virus (HBLV) or human herpesvirus-6; neither EBV nor HBLV has yet been shown to play a causal role in the illness.