The histologic, hematologic, and morphometric findings of 40 patients positive for the human immunodeficiency virus (HIV) were compared statistically with those of 40 patients with primary myelodysplastic syndromes (MDS) and those of 32 HIV-negative patients with infectious diseases. The severity of anemia and the abnormalities of erythropoiesis in the group of HIV patients were less pronounced than in the group with MDS; megakaryopoiesis showed similarities only with the group of patients with infectious diseases, and characteristics of dysplasia were not observed. Granulopoiesis in MDS showed an increase of blasts in several cases; this was not found in any biopsy specimen from the HIV group. In addition, a statistically significant increase of monocyte-like cells and giant bands could be observed in the bone marrow of the HIV patients. The peripheral blood findings and bone marrow picture in the series of our HIV patients appeared to be related mainly to the influence of opportunistic infections, although a direct effect of the HIV itself could not be excluded.