A 35 year-old man was diagnosed with a Primary Mediastinal Nonseminomatous Germ Cell Tumor, with initial differentiation toward a yolk sac derived neoplasm, and treated with classical PEB chemotherapy (Platinum, Epirubicin, Bleomicin). During treatment sequential CBCs showed a rapidly worsening thrombocytopenia (grade 4), with normal Hb level, moderate leucocytosis with absolute neutrophilia; the microscopic analysis of peripheral smears demonstrated dacriocytes, erythrobalsts, dysplastic neutrophils, rare myeloid precursors, monocytosis and a small population of atypical cells, accounting for 2-3% of total, so that neoplastic myelophthisis was suspected. The morphological analysis of bone marrow aspirate showed hypercellularity with almost absent megakaryocytes, slight red cell dysplasia and severe displasia of the white cell line. The same atypical cell population already seen on peripheral blood, accounted for around 25-30% of total cells. It consisted of large-size polinucleated cells, with thickened chromatin, often in mitosis, with wide cytoplasm, seldom with vacuoli, never clustering nor associating in syncytia, but randomly distributed in the interstitium (Figure 1). Although, in literature association of primary extragonadal germ cell tumors and hematological disorders -above all a rare entity such as acute megakaryoblastic leukemia- have been reported, to our knowledge bone marrow invasion by such a primary neoplasm has been more rarely described.