In order to determine whether the newly discovered human herpesviruses (HHVs) are involved in multiple sclerosis (MS), we investigated by polymerase chain reaction the presence of specific deoxyribonucleic acid (DNA) sequences belonging to human herpesvirus 6 (HHV-6) and to human herpesvirus 8 (HHV-8), in the peripheral blood mononuclear cells (PBMCs), and in the brain and spinal cord plaques from MS patients. Normal adult and stillborn children's brains were investigated as controls. PBMCs from 56 MS patients contained HHV-6 DNA in only 3 cases and in none were there HHV-8 sequences. The cerebral DNA from 5 MS patients was positive for HHV-8 and not for HHV-6 sequences, while the nervous tissue of one patient who died with neuromyelitis optica was positive for HHV-6 and negative for HHV-8. The brains of 4/8 adult controls were positive for HHV-6, as were 3/8 for HHV-8; none of the 7 stillborn children's cerebral tissue contained HHV-6 sequences, while 2 contained HHV-8 DNA. Although these data do not support a hypothesis that there is a role for these two HHVs in the pathogenesis of MS, nevertheless it may be suggested that (1) the two viruses possess strong neurotropism and the central nervous system seems to be a reservoir for them (2) HHV-6 infection is probably not transmitted maternally, but is acquired later in infancy.