We report here a very severe case of thymoma-related aplastic anemia that developed after thymectomy. The patient was a 50-year-old man diagnosed with myasthenia gravis. Chest CT showed thymoma measuring 7.5 cm in diameter, and extended thymectomy was performed. Irradiation to the anterior mediastinum was added postoperatively. Thirteen months after surgery, hemogram showed severe neutropenia: leukocyte count 0.32×10(9)/l with 11% neutrophils; Hb 10.7 g/dl; and platelet count 100×10(9)/l. Although cyclosporine (CSP, 5 mg/kg/day) was administered, dose reduction was necessary because of renal damage. Cytopenia deteriorated to a leukocyte count of 0.71×10(9)/l with 21% neutrophils; Hb 5.9 g/dl; and platelet count 24×10(9)/l. However, addition of antithymocyte globulin (ATG, 15 mg/kg) led to hematopoietic recovery of all three lineages within one month. He is clinically stable with no transfusion requirement after 22 months with CSP maintenance therapy. Although thymoma-related aplastic anemia has been reported to have an extremely poor prognosis, high efficacy of CSP has been reported recently. In our case, ATG in combination with CSP was efficient. Adequate immunosuppressive therapy seems to be important for the clinical management of these patients.