The records and histopathology of 4 patients with primary mediastinal seminoma were reviewed. All of them were male and at presentation had one or more poor prognostic factors, namely superior vena cava obstruction, hilar lymphadenopathy, bulky tumor, and raised alpha-fetoprotein titers. Their treatment consisted partly or entirely of chemotherapy. Three of them survived free of disease 2-5 years off treatment, 1 of them died of pulmonary fibrosis with no tumor found at postmortem examination. The prognostic factors and treatment of this tumor and the risk of combined radiotherapy and chemotherapy containing bleomycin are discussed.