To clarify long-term effects of intrathecal administration of interferon (IFN)-alpha in subacute sclerosing panencephalitis (SSPE), we followed up a patient with SSPE for 18 years, who had been treated successfully using intrathecal IFN-alpha with response-based dose adjustments. IFN-alpha therapy dramatically induced remission of disease and greatly improved quality of life for 7-8 years, but this was followed by severely deterioration with decorticate posturing and akinetic mutism. Thus, IFN-alpha-induced remission appears most likely to be temporary, even when an SSPE patient shows an excellent initial response. To improve long-term outcome for SSPE patients, more effective therapy is needed.