Ten patients with multiple sclerosis who were treated with human fibroblast interferon (IFN-B) for 6 months showed a significant reduction in their exacerbation rates compared with their rates before treatment (P < .01). The IFN-B was administered intrathecally by serial lumbar punctures. There was no significant change in the exacerbation rates of ten multiple sclerosis control patients before and during the period of observation. The IFN-B recipients have now been on the study a mean of 1.5 years, the controls, 1.2 years. The clinical condition of five of the IFN-B recipients and one of the control patients has improved, whereas the condition of five of the controls and one of the IFN-B recipients has deteriorated (P < .036). These findings warrant cautious optimism about the efficacy of intrathecal IFN-B in altering the course of multiple sclerosis and support concepts of a viral or dysimmune etiology of the disease.