Two cases of mediastinal germ cell tumors associated with primary hemoproliferative disorders are reported. The first case was a 23-year-old man who presented simultaneously with a mediastinal immature teratoma with focal yolk sac differentiation and a cytologically benign histiocytic proliferation associated with refractory thrombocytopenia. In the second case, an unsuspected mediastinal immature teratoma with focal yolk sac and hepatic differentiation was found postmortem in a 33-year-old man who had died of fulminant malignant histiocytosis. Although the histiocytosis in the former case failed to fulfill all the usual diagnostic criteria of malignant histiocytosis, both cases probably represent different manifestations of the same pathologic process. Review of the 19 published cases of hematologic malignancies associated with mediastinal germ cell tumors and of related experimental studies suggests a role for yolk sac differentiation in the pathogenesis of this syndrome.