Background: Opsoclonus-myoclonus ataxia is a rare neurologic syn drome, often paraneoplastic in origin, but reported in association with various infections. Little is known about adult-onset opsoclonus myoclonus syndrome (OMS) outside of individual case reports Objective: Describe a case of herpes-simplex virus 1 encephalitis presenting as opsoclonus-myoclonus ataxia Patients and methods: A 35 year-old woman, with no known comorbidities, developed a headache with nausea and vomiting, without fever or nuchal rigidity. In the following 48-hours, she presented an altered mental status, opsoclonus and myoclonus and was admitted to hospital. Lumbar puncture: 40 cells (60% mononu clear), protein 140 mg/dL and a normal glucose. Gram stain and culture for bacteria and fungi were negative. A PCR for herpes simplex virus was positive. Brain MRI: normal. Chest, Abdomen and Pelvis CT: no signs of neoplasia. Results: The patient received intra-venous acyclovir for 21 days, with resolution of symptoms Conclusion: OMS is an uncommon presentation of infections of central nervous system. Its fame extends further to the fact that OMS can be a harbinger of occult malignancy. The Adult-onset presentation is rare. Paraneoplastic and parainfectious causes (particularly virus) are common; however, more often OMS in adults occurs after systemic infection. After this report, HSV1 infection should be considered in OMS cases. We described the first case of OMS secondary to Herpes-Simplex Virus 1 infection.