A 51-year-old woman with a 3-year history of diplopia was admitted to our hospital. Repetitive stimulation of the right median nerve revealed decreasing hand muscle responses. Edrophonium chloride administration alleviated the symptoms. The serum antiacetylcholine receptor antibody level was 3.3 nmol/l; thus, myasthenia gravis was diagnosed. A chest roentgenogram revealed a left inferior mediastinal mass, and a chest computed tomographic scan revealed a fat density mass partly containing soft tissue areas in the left side of the anterior mediastinum. Median sternotomy and extended thymectomy were performed. The pathology examination of the tumor revealed mature adipose tissue including islands of thymic tissue containing Hassall's corpuscles, without germinal centers. The tumor was a thymolipoma of the anterior mediastinum. Her postoperative course was good, and she is doing well at 1 year postoperatively.