Three new cases of primary mediastinal seminoma are presented, and 124 other cases in the literature are reviewed. These tumors primarily affect white males in the third decade. Chest pain is the most common symptom, but 20 per cent of the patients can be asymptomatic. Testicular atrophy occurred in only 7 patients. In one of these a testicular seminoma developed seven years after treatment of a primary mediastinal tumor. Supraclavicular adenopathies were found in 10 patients. Chest x-ray films typically show an anterior mediastinal noncalcified mass with smooth lobulated borders. Twenty-eight of 74 surgical resections attempted were successful; 13 of these received adjuvant irradiation. Of 64 cases in which radiation therapy was the primary mode of treatment, local failures occurred only in 7 cases. These tumors spread primarily by distant metastases. Theories of tumoral origin and patterns of spread are discussed. A dose of 3,000 rad in three weeks and 4,500 rad in four and one-half weeks to the mediastinum and supraclavicular areas is recommended for minimal and bulky disease. Primary mediastinal seminoma is as radiosensitive as its testicular counterpart.