Background: Human herpesvirus 6 (HHV-6), a widesp- read virus and causative agent of exanthema subitum in children, has been asso ciated with a number of neurologic disorders including cranial nerve palsies, se izures, encephalitis, meningitis, and multiple sclerosis. Patient: A 31-year-old man presented with bilateral optic neuropathy, disc edema, and unilateral ton ic pupil, which were found to be associated with acute HHV-6 infection. The pat ient had been suffering from juvenile diabetes for 5 years. One week after onset of intravenous antiviral therapy with foscarnet, disc edema subsided, and tonic pupil reaction was no longer detectable. Conclusions: HHV6 infection may play a role as a causative agent in patients with optic neuropathy and tonic pupil. Background: Human herpesvirus 6 (HHV-6), a widesp-read virus and causative agent of exanthema subitum in children, has been asso ciated with a number of neurologic disorders including cranial nerve palsies, seizures, encephalitis, meningitis, and multiple sclerosis . Patient: A 31-year-old man presented with bilateral optic neuropathy, disc edema, and unilateral ton ic pupil, which were found to be associated with acute HHV-6 infection. The patient had been suffering from juvenile diabetes for 5 years One week after onset of intravenous antiviral therapy with foscarnet, disc edema subsided, and tonic pupil reaction was no longer detectable. Conclusions: HHV6 infection may play a role as a causative agent in patients with optic neuropathy and tonic pupil.