A 34-year-old male who has recieved regular hemodialysis three times a week for 2years due to chronic renal failure was admitted due to chest pain. Chest x-ray and computed tomography showed large sized anterior mediastinal mass compressing heart and great vessels posteriorly. Open lung biopsy was performed and the result was spindle cell type thymoma. We recommanded surgical resection but he refused and discharged. After 8 months, we observed marked regression of the thymoma without any other treatment. We report a case of spontaneous regression of thymoma in patient with chronic renal failure during regular hemodialysis.