Five patients with clinically advanced subacute sclerosing panencephalitis (SSPE) were given human leukocyte interferon (IFN) by the lumbar route, 1 million IU every other day for a total of 30 days. Intrathecal IFN produced a meningeal inflammatory reaction in all patients and was associated with transient hemiparesis in 1. It persisted in the cerebrospinal fluid at measurable levels for 48 hours after a single injection. Although improvement was temporally related to intrathecal IFN in 1 patient, it is not clear whether this was induced by IFN or a spontaneous remission. A randomized controlled trial would be necessary to evaluate IFN critically as a therapy for SSPE.