Bone marrow necrosis is defined by extensive necrosis of the myeloid tissue and bone marrow stroma. Diagnosis is done on characteristic cytological pattern of the bone marrow aspiration and/or biopsy. We report two observations. The first patient, aged 75, has been hospitalized for fever, asthenia and lower back pain. An haematological malignancy was suspected after observation of a few peripheral blood blast cells, but necrosis was found on the bone marrow aspiration and could not lead to further haematological diagnosis. Within next days, the white blood cell count and the number of blasts increased, leading to the diagnosis of acute monoblastic leukaemia. A chemotherapy was started but the patient died 20 days after admission. The second patient, aged 28, has been hospitalized for severe bleeding a few days after the diagnosis of a metastatic gastric tumour. The bone marrow aspiration, made for the evaluation of a thrombocytopenia, showed a massive necrosis. The patient deceased shortly after hospitalization. According to literature, bone marrow necrosis is in most instances secondary to either an haematological malignancy (60%) or to a solid tumour (30%), but only at times observed with a non-malignant disorder. Bone pain, fever, cytopenias and elevated serum lactic dehydrogenase and alkaline phosphatase are frequently reported, but are mostly non specific of the diagnosis in these malignant conditions. Examination of the bone marrow leads to the diagnosis: cells are pycnotic, scarcely recognizable in a background of amorphous extracellular eosinophilic proteinaceous material, and histology shows disappearance of fat spaces with preservation of the bone tissue. Tissue hypoxemia due to microcirculation failure may be the main mechanism leading to the necrosis, whatever the related disorder. Supportive care together with specific therapy of the causal disease must be started promptly. The prognosis depends on the underlying illness and is generally very poor when extensive necrosis is observed.