Primary pure cell seminoma of the mediastinum is a rare and potentially fatal lesion. Encroachment on or invasion of adjacent structures is common, as are distant metastases. The differentiation between pure cell seminomas and mixed germ cell tumors should be made since the extreme radiosensitivity of seminomas improves the prognosis. The presence of metastases from primary mediastinal seminomas, as with testicular germinomas, need not be a deterrent to long-term cure. Recommended therapy consists of total or subtotal removal, radiation therapy (2,500 to 3,500 rads delivered over 2 to 4 weeks), and, in some patients, chemotherapy. A patient who was successfully treated with subtotal removal and irradiation is presented.