Central nervous system (CNS) demyelination has been suggested to be associated with infections caused by the Epstein-Barr virus (EBV) or herpes simplex virus (HSV)-1. CNS inflammatory demyelinating disease (IDD) rarely presents as a large lesion. We evaluated samples of serum and cerebrospinal fluid (CSF) by enzyme-linked immunosorbent assay to detect recent infection with these viruses and analyzed CSF and brain specimens by polymerase chain reaction (PCR) or immunohistochemical studies for evidence of these viruses in three patients with biopsy-proven CNS IDD. The results of PCR tests for EBV and HSV in CSF or brain specimens were negative. Elevated anti-EBV or -HSV antibody levels were not found in serum or CSF in any patient. Immunohistochemical studies showed that IDD lesions were negative for latent membrane protein (LMP)-1, Epstein-Barr nuclear antigen (EBNA)-2, and EBNA noncoding RNA (EBER)-1. These results suggest a negative association between CNS IDD and EBV or HSV.