The effects of interferon-beta(1b) (IFN-beta(1b)) administration in multiple sclerosis (MS) patients have been confirmed, however, those in neuromyelitis optica (NMO) patients have not been shown. In this study, we assessed the effects of IFN-beta(1b) treatment on disease exacerbation and disability progression in MS or NMO patients.We reviewed a series of 104 consecutive patients with relapsing-remitting MS (RRMS) (69) or NMO (35) treated with IFN-beta(1b) in the MS clinical center of a national hospital in Japan.The relapse number in the RRMS patients significantly decreased within 1 year after IFN-beta(1b) treatment (p 0.05). The change in Kurtzke's Expanded Disability Status Scale score 1 year after treatment was higher in the NMO patients than in the RRMS patients (p = 0.0225).In NMO patients, IFN-beta(1b) treatment was not effective in reducing the relapse number and the disability progression.