An open therapeutic trial of isoprinosine was conducted in 15 patients with subacute sclerosing panencephalitis (SSPE). Long-term remissions occurred in 5 (33 percent), with documented improvement sustained for 2 or more years. Another patient was in remission 9 months after starting treatment, and three patients had transient remissions or stabilization. The disease was unaltered in five patients who had rapidly progressive SSPE when treatment started. These results compare with an average remission rate of about 5 percent in several series of untreated cases of SSPE or in cases treated with other antiviral agents. Patients in remission continued to have elevated cerebrospinal fluid (CSF) IgG and measles antibody titers, with one exception. Isoprinosine was tolerated for several years without side effects, except for mild hyperuricemia.