Teratomas are primary germ-cell tumours in the mediastinum. Although they are generally slow-growing and asymptomatic, rapid growth causing life-threatening complications can occur. Sebaceous secretion, insulin production, chorionic gonadotropin secretion and pancreatic enzyme secretion are the presumptive causes of tumour progression. Only few cases of rapidly growing teratomas have been reported previously. Here, we present a case of a giant mature teratoma in the mediastinum that presented with rapid growth and compare the characteristics of this case with those of previous cases.