Patients with Graves' orbitopathy have a higher probability of myasthenia gravis than does the normal population. Overlapping clinical features cause diagnostic confusion in such a situation. We herein report a patient with Graves' orbitopathy and myasthenia gravis (GO-MG) with normal left eyelid height, but in whom upper eyelid retraction was shown after edrophonium chloride administration. Upper eyelid retraction in GO-MG is occasionally masked by a myasthenia effect. The upper eyelid height must be carefully monitored in patients with Graves' orbitopathy to detect the presence of concomitant myasthenia gravis.