Isoprinosine (inosiplex), a proposed antiviral agent, was given to 18 patients with subacute sclerosing panencephalitis (SSPE). No great improvement occurred in any patient soon after treatment, but 2 patients did improve substantially several months after discontinuing Isoprinosine. Spontaneous improvement also occurred in 4 out of 96 controls. Because of the variable natural history of SSPE, differences in clinical improvement between treated patients and controls were not significant. Changes in serial measles antibody titers did not correlate with therapy.