To understand at what level hematopoiesis is impaired in AIDS patients, we quantitated bone marrow cells of the myeloid and lymphoid lineages in their different stages of differentiation. The bone marrow aspirates of 20 HIV-infected patients (18 with CDC IV) and 11 controls were analyzed by flow cytometry, using a panel of monoclonal antibodies. The mean percentages of CD34+ and CD 10+ hematopoietic precursor cells were not significantly altered in the HIV-infected patients. The values of these patients however scattered in a much wider range than in the control group, some patients showing extremely high values. Patients with high levels of CD34+ and CD 10+ precursor cells in the bone marrow had lower blood leukocyte counts and lower blood and bone marrow CD4 cell numbers than patients with a low percentage of CD 10+ and CD34+ cells (p < 0.01). The percentages of the CD34+ cell subsets studied (CD34+CD33+, CD34+CD14+, CD34+CD15+) were not altered. Bone marrow CD4+ lymphocytes, especially those of CD45RA- phenotype, were reduced, while the CD8+ cells (especially HLA-DR+/CD45RA+ cells) were increased. These results show that the hematologic disorders in AIDS are not simply caused by an altered number or composition of bone marrow precursor cells. We postulate that hematopoiesis is inhibited due to an imbalance in the bone marrow T lymphocyte populations.