Thymoma is the most common tumor in the anterior mediastinum. A 56-year-old man presented unremitting and periodic chronic diarrhea of 9 weeks duration, and clinical examination revealed a huge nonhomogeneous mass lesion in the right lung and leukocytosis. He was treated with CHOP regimen (cyclophosphamide 1,200 mg/m(2), doxorubicin 50 mg/m(2), vincristine 1.5 mg/m(2), and prednisolone 75 mg/m(2) × 5 days) based on lung mass computed tomography-guided biopsy, but he was reevaluated because neither symptom improved. Surprisingly, celiac disease was documented with increased titer of immunoglobulin antibodies to gliadin and tissue transglutaminase. Lung mass rebiopsy and thymectomy demonstrated thymoma. After surgery, the patient showed aplastic anemia that responded well to cyclosporine. At 2-year follow-up, the patient's hematologic status and diarrhea were completely recovered and no symptom and/or sign of thymoma recurrence was seen.