Inosiplex was administered to 98 patients with subacute sclerosing panencephalitis (SSPE) in the United States and Canada for variable periods of time up to 9.5 years. Survival data from these 98 patients were compared by life-table analysis with survival in three SSPE control groups drawn from SSPE patients contracting the disease in Israel, Lebanon, or the United States at about the same time as the inosiplex-treated patients but treated differently or not at all. In the inosiplex-treated patients the actuarial probability of survival at 2, 4, 6, and 8 years from onset of SSPE was 78%, 69%, 65%, and 61%, compared with 38%, 20%, 14%, and 8% in a composite control group (p less than 0.01 for all four comparisons). Statistical adjustments for time-to-treatment bias did not affect this result: a modified logrank procedure demonstrated that the risk of dying in the treatment group was 43% of that in the controls. Inosiplex seems to be able to prolong life in patients with SSPE.