Currently there is no clearly defined, effective treatment of patients with HAM/TSP. Corticosteroids are the mainstay of therapy but provide mostly only transient symptomatic relief. Treatment with human interferon has been shown to improve acute and chronic hepatitis through its anti-viral and cytostatic effects. Further, patients with relapsing-remitting multiple sclerosis, a disease thought to be at least in part a T cell mediated, immunopathological process, exhibit a marked reduction of the frequency of new lesion formation while on this medication. This latter effect may in part be explained by an anti-inflammatory effect of interferon which allows repair of the blood brain barrier. To evaluate possible role interferon beta in the treatment of HAM/TSP, we studied its effect on induction on regulatory factors and spontaneous in vitro proliferation of peripheral blood lymphocytes (PBLs) from HTLV-I infected individuals. Recombinant interferon beta-1b inhibited spontaneous proliferation of PBLs from asymptomatic HTLV-1 carriers and patients with HAM/TSP in a dose dependent manner and induced expression of interferon regulatory factor-2 which is associated with down regulation of proinflammatory cytokines such as interferon gamma. These preliminary results indicate that treatment with interferon-b may ameliorate the immune dysregulation induced by HTLV-1 and may have therapeutic effect in the treatment of HAM/TSP.