Optic neuritis and myelitis are manifestations in both multiple sclerosis (MS) and neuromyelitis optica (NMO). But unlike MS, NMO is characterized by severe optic neuritis, longitudinally extensive and transverse myelitis, and the presence of aquaporin-4 antibody. Since patients with optic neuritis and myelitis have often been diagnosed with "optic-spinal MS (OSMS)" in Asia, it was obscure whether "OSMS" is synonymous with NMO or includes both NMO and MS. Interferon β (IFNβ)-1a and -1b are used as the first-line disease-modifying therapy for MS. However, some neurologists have been reluctant to use IFNβ to treat patients with optic-spinal symptoms, because IFNβ therapy is not efficacious in NMO. To evaluate the therapeutic effect of IFNβ in patients with "genuine" OSMS, we retrospectively evaluated Japanese MS patients who fulfilled the following six criteria: 1) Relapsing-remitting MS with optic-spinal presentation alone (no brain symptoms), 2) With or without asymptomatic brain MRI lesions, 3) Oligoclonal IgG band-positive, 4) aquaporin-4 antibody seronegativity, 5) No myelitis extending longitudinally over ≥ 3 vertebral segments, and 6) Duration of IFNβ-1b therapy ≥ 2 years. Among 157 patients with MS, six (four women and two men, age 43.8 ± 8.5 years old) met all the criteria. Their Expanded Disability Status Scale scores were lowered (4.1 ± 2.4 → 3.1 ± 2.8) (P = 0.033) and annualized relapse rate was decreased (0.59 ± 0.34 → 0.13 ± 0.15) (P = 0.027) after IFNβ-1b therapy. These results suggest that IFNβ is therapeutically effective in inhibiting functional worsening and reducing relapse rate in "genuine" OSMS.