Twenty-five patients with a malignant germ cell tumor of the mediastinum were treated at the National Cancer Center Hospital, Tokyo. Three patients had pure seminomas while 22 had a nonseminomatous histology. The treatment modalities consisted of surgery alone in 7 patients, surgery with chemotherapy and/or radiation in 14, and chemotherapy and/or radiation without surgery in 4. Cisplatin-based chemotherapy was administered in nine patients, one of whom successfully underwent high-dose chemotherapy with autologous bone marrow transplantation after resection of the tumor. A complete response was achieved in only five patients. The overall survival rate at 5 years was 33.5% and the median survival time was 51 weeks. Eight patients (32%), all of whom underwent surgery either with or without chemotherapy and/or radiation, were free of disease with a mean follow-up period of 439 weeks (72-1,120 weeks). The median survival times for 14 patients undergoing combined modality treatment, 7 patients undergoing surgery alone, and 4 patients undergoing chemotherapy and/or radiation were 83, 16, and 18 weeks, respectively.