In order to clarify the clinical characteristics and effects of acetylcholinesterase inhibitors of patients with generalized myasthenia gravis with antibodies to muscle specific kinase (MuSK), we investigated seven patients with MuSK antibodies and eleven patients without both antibodies of acetylcholine receptor and MuSK. All patients with MuSK antibodies showed bulbar symptoms, which frequency was significantly higher compared to those in patients without double antibodies. The frequency of positive result of Tensilon test was significantly lower in patients with MuSK antibodies than in those without double antibodies. In response to intravenous edrophonium chloride, MuSK positive patients showed adverse reactions in a small dosage of edrophonium chloride, less than 5 mg, such as fasciculation on facial muscles and stuffy sensation of throat. The adverse responses to a small dosage of intravenous edrophonium chloride injection is useful information to distinguish patients with seronegative generalized MG, whether they have MuSK antibodies or not. When acetylcholinesterase inhibitors medication is tried to patients with MuSK antibodies, if necessary, a small dosage of inhibitors should be used to avoid cholinergic hypersensitivity.