In 25 patients (22 males, 3 females--median age 39 years) with AIDS (CDC stages IV A-D) and no preceding myelotoxic therapy, morphometry and immunohistochemistry (CD 61-Y 2/51) was performed on trephine biopsies of the bone marrow to evaluate the megakaryocytic lineage. In comparison with megakaryocytes in the myelodysplastic syndromes (MDS) significant differences were evident. In AIDS this cell population revealed a size distribution within the normal range (control group) and no predominance of micromegakaryocytes characteristic for MDS. Furthermore, by determination of the form factors more irregular shapes of cell and nuclear perimeters could be shown. Finally, a not-evaluated number of precursors (promegakaryoblasts) was calculable. Particularly in those patients (n = 15) with AIDS-related severe thrombocytopenia the missing increase in the relative amount of promegakaryoblasts was conspicuous. This result was strikingly different from findings in idiopathic (autoimmune) thrombocytopenia and suggested an impairment of progenitor cell proliferation and differentiation in the acquired immunodeficiency syndrome. In conclusion, morphometry in combination with immunohistochemistry failed to establish characteristic myelodysplastic aspects of the megakaryocytic lineage in AIDS. For this reason, bone marrow lesions in this disorder should be properly termed HIV-myelopathy and not myelodysplasia.