We describe a 22-year-old woman with an adult-onset, slowly progressive form of subacute sclerosing panencephalitis (SSPE), who was repeatedly evaluated by brain magnetic resonance spectroscopy (MRS). The brain lesion spectrum showed a decrease in N-acetylaspartate (NAA) resonance, an increase in inositol (Ins) resonance, and an unaltered choline signal. These findings suggest neuronal loss and reactive gliosis without inflammation, consistent with brain biopsy findings showing astrocytic proliferation unaccompanied by lymphocytic infiltrates. The unusually protracted clinical course might be attributable to an absence of inflammatory infiltrates in the brain. Intraventricular interferon injection plus oral inosine pranobex treatment produced a substantial improvement in the MRS findings, suggesting the validity of monitoring MRS in SSPE.