The objective of this study was to investigate the ultrastructural characteristics of nucleated cells in the bone marrow of patients with aplastic anemia (AA). This was done by observing the morphology of nucleated cells in bone marrow aspirates from 20 patients with AA by transmission electron microscopy. Erythroblasts were decreased in all cases and not observed in 6 cases. Nuclear abnormalities, such as pyknosis, karyolysis, karyorrhexis, apoptosis, and "Swiss cheese"-like changes, were found in 10 cases. Focal cytoplasmic necrotic changes and cytolysis were found in 3 cases. There were more megaloblasts in 4 cases. Abnormalities of granulocytes were found in 12 out of 18 cases. Megakaryocytes showed focal cytoplasmic necrotic changes. Most monocytes had dendritic features, including excessive cytoplasm, processes, and large round nuclei in all cases. Other monocytes illustrated typical monocytic features with twisted nuclei, plentiful RER, vacuoles, lysosomes, and prominent Golgi apparatus. Macrophages and hemophagocytes occurred in all cases. The incidence of lymphocytes was high in 17 out of 20 cases and occasionally lymphocytes were enlarged in 8 cases. More plasmacytes and plasmacytoid lymphocytes were found in 5 and 3 cases, respectively. The observations suggest that (1) the universal nuclear injury of erythroblasts may be related to the pathogenetic pathway of AA development; (2) the dendritic cells and hemophagocytes from the mononuclear phagocyte system may play a more critical role in hematopoietic failure of AA, directly and/or indirectly; and (3) besides T lymphocytes, increasing numbers of plasmacytes or plasmacytoid lymphocytes are associated with AA in some cases.