Peripheral blood mononuclear cells and plasma of 113 patients with neurological disorders and 150 blood donors were analyzed for HHV-6 and HHV-7 sequences by PCR. The prevalence of HHV-6 was significantly higher in patients with multiple sclerosis (P < 0.01) than in cases of nondemyelinating diseases of the central and demyelinating diseases of the peripheral nervous systems and blood donors. HHV-6 viremia was found only in patients with multiple sclerosis, predominantly in the active phase of the disease. A significantly higher frequency of HHV-7 reactivation in patients with demyelinating diseases of the peripheral nervous system suggests also its association with demyelinating processes.