Twenty-nine young men (mean age 29 years) had primitive neuroectodermal tumors (PNETs) arising in germ cell tumors (GCTs). Nine patients had PNETs confined to the testis, eight patients had PNETs in the testis and at metastatic sites, and 12 patients had PNETs identified only at extratesticular sites. Immunohistochemistry was of use in the further classification of these PNETs as neuroblastoma, medulloepithelioma, peripheral neuroepithelioma, or ependymoblastoma. The histologic pattern of PNETs in the testis (neuroblastoma or medulloepithelioma) did not predict which tumors metastasized. PNETs localized to the testis did not affect prognosis. Eight patients with no PNETs outside the testis were free of disease 1 month to 10 years after diagnosis. PNETs in extratesticular sites were an adverse prognostic factor. Nineteen patients with extratesticular PNETs had adequate clinical follow-up. Thirteen are dead of disease from 4 months to 5 1/2 years (mean 26 months) after diagnosis, four are alive with disease 6 months to 2 years after diagnosis, and two have no evidence of disease with short follow-up (6 and 17 months). Mean survival was longer (34 months) for patients whose extratesticular PNET was neuroblastoma than for those with other types of PNETs (13 months). Chemotherapy directed against GCTs was not effective in patients who developed metastatic PNETs of GCT origin. We conclude that extratesticular PNETs in patients with testicular GCTs are usually fatal, but patients with neuroblastomatous metastases may have a more prolonged course.