Chronic fatigue syndrome (CFS) is characterized by chronic, debilitating fatigue lasting greater than 6 months. Frequent chronic and recurrent findings include 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 laboratory 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, varying levels of antithyroid antibodies, partial hypergammaglobulinemia, elevated CD4:CD8 ratio, decreased cytolytic activity of natural killer cells, and 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. Preliminary evidence suggests that many of these features of CFS also are seen in patients with fibromyalgia.