We describe the clinical characteristics and prognostic features of 71 patients with advanced poorly differentiated carcinoma of unknown primary site. These patients had at least one component of the extragonadal germ cell cancer syndrome that we have previously described. Of 68 patients who received therapy, 62 were given intensive cisplatin-based combination chemotherapy that is used for treatment of germinal neoplasms. Fifteen patients (22%) had complete responses, and 9 patients (13%) have remained free of tumor after a minimum follow-up of 36 months (range, 36 to 67 months). Tumor in the mediastinum, retroperitoneum, and lymph nodes was associated with a favorable outcome of treatment when compared with tumor in other locations (p = 0.0016, Cox regression analysis). Although the histogenesis of tumors in many of these patients remains unclear, we believe the tumors most likely originate from germ cells. Patients with advanced poorly differentiated carcinoma should be considered for treatment with cisplatin-based combination chemotherapy, particularly if tumors occur predominantly in the mediastinum, retroperitoneum, or lymph nodes.