We report the case of a 53-year-old female patient, who developed bilateral blepharoptosis, limb weakness, dysphagia, and dyspnea several days after human natural interferon-beta (IFN-beta) treatment for chronic active hepatitis C. A positive edrophonium test, an elevated anti-acetylcholine receptor antibody titer, and decrements in the amplitude of muscle action potentials evoked by repetitive stimulation confirmed the diagnosis of myasthenia gravis (MG). Since she had been suffering from drooping of her right eyelid, fluctuating diplopia and easy fatiguability of limbs before receiving IFN-beta, her symptoms of MG were considered to be exacerbated by IFN-beta. It is recommended that IFN-beta should be used with particular care in patients with known MG or its compatible symptoms.