Mediastinum is a common site where benign tumors, like teratomas, can develop. Usually, these lesions do not cause any symptoms and the diagnosis is reached accidentally. As they enlarge they may cause symptoms by compressing the nearby structures of the thorax, mostly the trachea and the bronchi. Extrinsic compression of the heart or the great vessels appears to be a very rare occurrence. Atrial fibrillation as the first clinical presentation of left atrial compression by a giant mediastinal teratoma is extremely uncommon and very few cases have been described in the English literature.