A case is presented of a nine-month-old male infant who presented acutely with an anterior neck and mediastinum mass compressing the trachea. Radiological assessment by ultrasound, magnetic resonance imaging and computed tomography suggested the nature of the mass to be a lymphatic malformation (cystic hygroma). The excised specimen was revealed as a mature teratoma. The inability to distinguish between lymphatic malformation and teratoma on multi-imaging modalities confers a risk of mismanaging these lesions with the use of sclerotherapy; surgical excision is the necessary treatment. The potential consequences are discussed.