IgM serum antibodies to Epstein-Barr virus are uniquely present in a subset of patients with the chronic fatigue syndrome. 
A unique subset of patients with chronic fatigue syndrome (CFS) and IgM serum antibodies to cytomegalovirus (HCMV) non-structural gene products p52 and CM2 (UL 44 and UL 57) has been described.  Fifty-eight CFS patients and 68 non-CFS matched controls were studied.  Serum antibodies to EBV viral capsid antigen (VCA) IgM and EBV Early Antigen, diffuse (EA, D) as well HVCMV(V), IgM and IgG; VP (sucrose, density purified V); p52 and CM2 IgM serum antibodies were assayed.  Mean age of CFS patients was 44 years (75% women).  Control patients were 9 years older (73% women).  Serum EBV VCA IgM positive antibody titers were identified in 33 CFS patients (Group A subset EBV VCA IgM 62.3+/-8.3, neg.  <20), but were not present in other CFS patients, (Group B subset EBV VCA IgM 6.8+/-0.7) controls (p<0.0001).  EBV VCA IgM titers remained positive in CFS patients from Group A for 24-42 months.  Serum antibody to EBV VCA IgM may be a specific diagnostic test for a second subset of CFS patients.