Cimetidine, an H2 histamine receptor antagonist, was used in subacute sclerosing panencephalitis patients for its immunomodulatory effect. Patients were randomly assigned to cimetidine (n = 7) and placebo (n = 7) groups. Neurologic disability index, lymphocyte functions, cerebrospinal fluid measles antibodies and IgG index were evaluated before and after 2 months of treatment. The neurologic disability index of the cimetidine group remained stable during the study period whereas the placebo group worsened. There were no differences in the immunologic test results, cerebrospinal fluid measles antibody titers and IgG index of the two groups. This study suggests that cimetidine may have a favorable effect on the clinical progression of subacute sclerosing panencephalitis. Further studies are required to investigate its mechanism of action and the associated changes in immune status.