Herpes simplex virus encephalitis (HSVE) is associated with a high mortality rate and a high probability of neurological sequelae. Good results are obtained when HSVE is promptly diagnosed and treated with acyclovir. We present a 71-year-old woman with clinically diagnosed HSVE, confirmed by PCR detection of HSV-1 DNA in the cerebrospinal fluid. She was treated with acyclovir (30 mg/kg day) for two weeks. Clinical and neuropsychological assessments 6 months after admission were normal; however MRI at 2, 6 and 12 months showed progressive deterioration with extensive white matter and cortical damage. Imaging studies of a cohort of patients surviving PCR-confirmed HSVE are needed to determine whether this pattern is occasional or a frequent form of progression.