Neuromyelitis optica (NMO) is a relapsing neurologic disease characterized by severe optic neuritis and transverse myelitis. A disease-modifying therapy for NMO has not been established. We retrospectively analysed the effect of low-dose corticosteroid (CS) monotherapy on the annual relapse rate in nine patients with NMO. We divided the clinical course in each patient into two periods; the CS Period in which CS was administered, and the No CS Period in which CS was not administered. Periods related to other immunological therapies, such as high-dose methylprednisolone, immuno-suppressants, interferon -beta, and plasma exchange, were excluded. As a result, the annual relapse rate during the CS Periods [median, 0.49 (range, 0-1.31)] was found to be significantly lower than that during the No CS Periods [1.48 (0.65-5.54)]. As for the dose of CS, relapses occurred significantly more frequently with '10 mg/day or less' than with 'over 10 mg/day' (odds ratio: 8.75). The results of the present study suggest a beneficial effect of low-dose CS monotherapy in reducing relapses in NMO.