We have measured the levels of antibody (ab) against different Epstein-Barr virus (EBV) antigens in 10 AIDS patients, 7 HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients and 20 control subjects. We found comparable serum levels of anti-EBV ab between AIDS patients, HAM/TSP patients and control subjects. By contrast, anti-EBV ab were present in the large majority of CSF from AIDS patients (70%) and HAM/TSP patients (60%) but only in 15% of control group. Our results support a synergistic role of EBV in retroviral infections of the central nervous system.