Extragonadal and poor risk nonseminomatous germ cell tumors. Survival and prognostic features. One hundred forty-nine patients with poor risk nonseminomatous germ cell tumors (NSGCT) treated between 1975 and 1988 were studied. Patients were considered poor risk if they had an extragonadal primary site or testicular NSGCT with low predicted probability of achieving a complete response (CR). Primary sites were the testis (99 patients), retroperitoneum (18 patients), and mediastinum (32 patients). Patients with mediastinal NSGCT had lower human chorionic gonadotropin (HCG) (P less than 0.0001) and lactate dehydrogenase (LDH) levels (P less than 0.0001), and more frequent yolk sac elements (P = 0.002). CR rates were 38% for mediastinal, 61% for retroperitoneal, and 38% for testicular primary sites. Mediastinal NSGCT patients more frequently required resection of residual malignancy to attain a CR (6 of 12). Mediastinal NSGCT had the worst event-free survival (P = 0.02). Cox regression analysis identified brain or liver metastases as the most important predictor of event-free survival in poor risk patients. Retroperitoneal NSGCT often have a poor outcome due to advanced presentation, but the likelihood of a CR to therapy can be predicted using criteria applicable to testicular primary tumors. Therefore, not all retroperitoneal NSGCT are poor risk, and retroperitoneal tumors are probably of occult testicular origin. Mediastinal NSGCT have distinct clinical and pathologic features, do not respond as well to chemotherapy, relapse more frequently, and have the worst survival. The likelihood of a CR cannot be predicted using criteria developed for primary testicular tumors, suggesting that mediastinal primary NSGCT is a distinct clinical entity. 