The majority of human immunodeficiency virus (HIV)-seropositive patients develop bone marrow abnormalities associated with hematopoietic malfunction during the progression of disease. One important manifestation of HIV-associated hematopoietic dysfunction is that after myelosuppression, bone marrow recovery, a process known to be mediated in part by the production of stromal cell-derived hematopoietic growth factors, is impaired. We sought to test the hypothesis that bone marrow stromal cells are infected by HIV-1 in vivo and that production of certain stromal cell-derived hematopoietic growth factors is deficient as a consequence. In this report, we demonstrate that bone marrow microvascular endothelial cells (MVEC), a key element of the stroma, are the predominant cells infected by HIV (5% to 20%) in bone marrow stromal cultures obtained from 11 consecutive HIV-seropositive patients. Although HIV-infected stromal cultures enriched for MVEC constitutively express normal levels of interleukin (IL)-4, IL-6, granulocyte (G)-colony-stimulating factor (CSF), granulocyte-macrophage (GM)-CSF, tumor necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta, and Steel factor, IL-1 alpha-induced release of IL-6 and G-CSF is significantly reduced in these cultures. These observations suggest that HIV infection of bone marrow MVEC reduces the capacity of hematopoietic stroma to respond to regulatory signals that normally augment blood cell production during periods of increased demand.