Naked megakaryocyte nuclei (NMN) were studied retrospectively in 100 bone marrow biopsy sections and aspirates from patients with acquired immunodeficiency syndrome (AIDS) and 124 non-AIDS controls. An NMN score was developed to semiquantitate the NMN. The AIDS bone marrows had an incidence of 97% and a mean NMN score of 2.24 which was a higher incidence and score than the control groups and was statistically significant. The control groups consisted of 20 chronic myeloproliferative disorders (incidence of 70% and score of 1.4); 24 cases of acute myelogenous leukemia and myelodysplastic syndrome (incidence of 29.2% and 0.46 score); 22 cases of non-AIDS immune thrombocytopenic purpura (incidence of 50% and 0.82 score); 11 cases of acute lymphocytic leukemia (0% incidence); 21 cases of anemia (52.4% incidence and 0.7 score); and 26 cases of lymphoma staging (35% incidence and 0.35 score). It is concluded that NMN are a very frequent and, with a high score, a specific finding in AIDS bone marrow examinations.