Herpes simplex (HSV) encephalitis is one of the most common central nervous system (CNS) viral infections in adults. Early diagnosis is essential for treatment. We describe the case of a 70-year-old man who reported sudden bilateral reduction of visus. Four days after admission the patient showed high fever, followed the next day by a generalised convulsive crisis and coma. A first magnetic resonance imaging (MRI) showed no alterations, whereas the second showed the usual patterns of HSV encephalitis. With a clinical suspicion of herpetic encephalitis an intravenous therapy with acyclovir was established. The diagnosis of herpetic encephalitis was confirmed by cerebrospinal fluid (CSF) detection of herpes simplex DNA sequences. A further ten days later we performed a third MRI, demonstrating the typical pattern of HSV encephalitis and an increase in size and signal of the optic chiasm.