Tetrofosmin is a ligand that forms a lipophilic, cationic complex with Tc-99m. Tc-99m tetrofosmin was developed as a myocardial perfusion imaging agent and also used to depict tumors. Mediastinal tumors is also detected by Tc-99m tetrofosmin. We report a case of extracardiac mediastinal activity detected by Tc-99m tetrofosmin scintigraphy, which revealed thymoma.