A rare case of Castleman's disease with myasthenia gravis is reported. A 55-year-old woman with bilateral ptosis, speech impairment, and severe dyspnea had been previously diagnosed with myasthenia gravis. Computed tomography showed a 5 cm × 3 cm paratracheal mass in the mediastinum, thought to be an ectopic thymoma. Two days after surgical resection, the patient suddenly developed dyspnea. Postoperative myasthenic crisis was diagnosed, and plasmapheresis was performed. Her general condition improved, and her subsequent course was uneventful. The final pathological diagnosis was mediastinal solitary Castleman's disease, hyaline vascular type. Castleman's disease with myasthenia gravis is especially rare. One of the serious complications is postoperative myasthenic crisis. For patients with myasthenia gravis, the rate of postoperative myasthenic crisis seems significantly higher in Castleman's disease patients than in patients with thymic epithelial tumors. Castleman's disease with myasthenia gravis is discussed along with a review of the literature.