Multiple sclerosis (MS) is rare in children and the efficacy and safety of interferon beta 1b (IFN-beta 1b) treatment in childhood MS has not yet been established. We started to treat a boy who suffered from relapsing remitting MS with IFN-beta 1b at 8.5 years of age, because he had severe neurological disability in consequence of frequent relapses and incomplete remission. After initiating IFN-beta 1b treatment, his clinical course moved to secondary progressive (SP) MS, and he demonstrated poor improvement in degenerative progression and his disability continued to worsen. We could speculate that IFN-beta 1b was not effective regarding the degenerative component of childhood MS as in that of adult MS. SPMS is extremely rare in children, and this case would provide a very important implication to predict the tolerability of IFN-beta 1b treatment depending on the type of clinical course in childhood MS.