Follow-up observations on patients with multiple sclerosis who were treated with human fibroblast interferon (interferon beta) administered intrathecally for six months revealed a persisting beneficial effect in terms of a reduction in exacerbation rates. At the time of our last report in 1982, ten interferon beta recipients had shown a reduction in their mean exacerbation rate from 1.8/yr before the study to 0.2/yr during the study while ten control patients with multiple sclerosis showed no change in their rates during the study (0.69/yr) compared with before it (0.68/yr). That report was based on observations made for means of 1.9 years in the recipients and 1.6 years in the controls. The recipient patients have now been followed up for 4.4 years (mean) and their exacerbation rates have continued to decrease to a current mean level of 0.16/yr. The control patients were "crossed over" and began receiving interferon beta intrathecally after they had been in the study for two years without showing any change in their rate. During the 2.0 years since crossover they also have shown a reduction in exacerbation rate to a mean of 0.30/yr. The toxic side effects of interferon beta administered intrathecally were acceptable in view of the benefit achieved. Interferon was identified in the cerebrospinal fluid (but not the serum) of two patients prior to treatment, which is probably a manifestation of de novo production of interferon by the central nervous system in response to the multiple sclerosis disease process.