We present here the case of an 11-year-old boy with herpes simplex encephalitis diagnosed on the basis of clinical features, serology, and response to acyclovir, who relapsed after 3 weeks of therapy. In vivo proton magnetic resonance spectroscopy (1H MRS) of the brain, at 8 and 16 weeks after the onset of symptoms, showed abnormalities, most prominently a reduction in the N-acetylaspartate/choline ratio. The role of 1H MRS in assessing disease activity is discussed.