Several studies have suggested a possible association of human herpesvirus-6 (HHV-6) with multiple sclerosis (MS), a demyelinating disease with a variable course and progression. To determine whether HHV-6 could be detected in the sera of CSF of patients with different subtypes of MS, we performed nested polymerase chain reaction (PCR) on samples obtained from MS patients as well as samples from normal adults or individuals with other neurological diseases. Ninety-six serum samples from 24 patients with MS, including 13 individuals with relapsing remitting MS, one individual with primary progressive MS, seven individuals with secondary progressive MS and three individuals with an unspecified type were analyzed. Multiple serum samples were examined from individuals over varying periods of time and included samples obtained during exacerbations, remissions, and at different stages of progressive disease. HHV-6 DNA was detected only in one out of 15 serum samples that were collected over a number of years from one individual with secondary progressive MS. No HHV-6 DNA was detected in CSF from six patients with MS or 14 patients with other neurologic disease. These results indicate that the presence of HHV-6 DNA in the serum or CSF of patients with MS is not a common phenomenon, at least within the limits of the sensitivity of our assay.