There are few transmission electron microscopic studies on bone marrow biopsies of patients with hematological disease owing to the difficulty of overcoming the artifacts of decalcification. Following the fixation of bone marrow biopsies thoroughly before a mild decalcification procedure, ultrastructural studies were performed on 13 patients with varied hematological diseases. Notable features included blood cell disorganization, fibroblast activation, myofibroblast transformation, as well as accumulation of collagen and extracellular amorphous matrix. In addition, excessive blood cell death in leukemia, apoptosis, and macrophage phagocytosis in myelodysplastic syndrome and polycythemia vera, as well as degranulation of eosinophils and megakaryocytes in chronic idiopathic myelofibrosis were predominant, respectively. The observations suggest that polyclonal fibroblast proliferation and extracellular matrix accumulation may result from inflammation resulting from excessive cell death and active material release of blood cells in the bone marrow of patients with hematological disease.