Three men with primary mediastinal germ-cell tumors subsequently developed a malignant hematologic disorder characterized by pancytopenia and marrow infiltration with hematopoietic blast cells. Two of these patients were classified as having acute megakaryocytic leukemia and the third was believed to have a myelodysplastic syndrome with a prominent megakaryocytic component. Analysis of clinical characteristics of these patients and review of the literature suggest that the proximate association of mediastinal germ-cell tumors with malignant hematologic disorders is neither a coincidence nor a consequence of chemotherapy given for the germ-cell tumor. We believe this association represents the evolution of a neoplastic disorder that initially involves a totipotent germ cell. These germ cells, when located in the mediastinum, apparently acquire hematologic phenotypes and are manifested clinically as a hematologic malignancy.