A 21-year-old male was admitted for evaluation of a mass shadow on chest film. On chest computed tomography showed 5 cm sized homogeneous low density based on the second thoracic vertebral body in the posterior mediastinum. The patient had been performed thoracic sympathectomy 6 months before admission and oxidized cellulose was used for hemostasis at that operation. Surgical resection was performed and microscopic result was foreign body granuloma caused by oxidized cellulose. Oxidized cellulose is an absorbable sterile mesh and used to control capillary or venous bleeding. Although the manufacturer recommends its removal after hemostasis is achieved, in clinical practice it is usually left in situ to reabsorb spontaneously, usually with no untoward effect.