An unusual case of myasthenia gravis is presented. The disease was clinically established in a 71-year-old Japanese woman by an electromyographic, pharmacologic test and anti-acetylcholine receptor (anti-AChR) antibodies demonstrated in the circulating blood. The coexistence of Hashimoto's thyroiditis was diagnosed by the presence of a diffuse thyroid enlargement, anti-thyroglobulin and anti-microsomal antibodies and a lowered thyroidal 131I uptake. A delusion of persecution developed during the treatment with pyridostigmine and ephedrine chloride, but the symptom disappeared after discontinuing the use of ephedrine chloride.