Six patients with primary seminoma of the mediastinum were treated at the Cancer Control Agency of British Columbia during the period from 1970 to 1982. Primary treatment used was external beam radiotherapy. Only one patient received both radiotherapy and chemotherapy. Tumor dose ranged from 3000 cGy in 16 daily fractions in 3 weeks to 3500 cGy in 20 daily fractions in 4 weeks using parallel opposed fields on megavoltage machines. Two patients received additional small-volume boost to the superior mediastinum, delivering 500 cGy and 1000 cGy, respectively. All patients had complete resolution of the mediastinal mass after treatment and are alive and free of disease. One patient developed a metastasis to the fifth lumbar vertebra 1 year after treatment. This metastatic lesion was treated with radiotherapy and the patient is alive and free of clinical disease. Mediastinal seminoma is radiosensitive and radiocurable.