Primary immature mediastinal teratoma is very rare. The prognosis of this tumour observed in the late teens or adulthood was thought to be very poor. However, recent advances in chemotherapy have improved the prognosis. Eleven patients > 15 years of age with immature mediastinal teratoma (two of our patients and nine from the literature) were reviewed. These patients were classified into survivor and non-survivor groups, and the characteristics in each group were evaluated. In the survivor group, the tumour was completely resected in all patients. In addition, patients showing a high level of alpha-fetoprotein were treated by pre-operative chemotherapy, and those who were pre-operatively negative for tumour markers were treated by post-operative chemotherapy and survived for a long period. All patients in whom the tumour was resected incompletely, or where no chemotherapy was performed, were included in the non-survivor group. In patients > 15 years of age with immature mediastinal teratoma, complete surgical resection of the tumour is indispensable. When complete resection is combined with chemotherapy, long survival can be expected. Conversely, unless both treatment methods are performed, the outcome is very poor.