The pathogenesis of peripheral blood cytopenias in AIDS patients is clearly multifactorial. Among the various contributing mechanisms, those involving a direct role of HIV-1 have been actively investigated in the past few years. It has now been convincingly demonstrated that HIV can impair the survival/proliferative capacity of purified haematopoietic progenitor cells. Although a subset of haematopoietic progenitor cells are perhaps susceptible to HIV-1 infection, both in vitro and in vivo, the suppressive effect does not require either active or latent infection and is probably mediated by the interaction of viral or virus-associated proteins with the cell membrane of haematopoietic progenitor cells. Both the viral load and the biological characteristics of the virus play an important role in suppression, since different isolates displayed different inhibitory activity. Haematosuppression is not a specific property of monocytotropic versus lymphocytotropic or low-replicating versus high-replicating isolates, and it will be important to exactly establish which viral component is crucial to suppression of haematopoietic progenitor cells. Since the haematopoietic stem cell is the common progenitor to both the myeloid and lymphoid lineages, the capacity of HIV to impair the growth of early haematopoietic progenitor cells could contribute not only to the frequent occurrence of anaemia, granulocytopenia and thrombocytopenia in AIDS patients, but also to the inability of the bone marrow to reconstitute a functional pool of mature CD4+ T-cells. It is also possible that haematopoietic progenitor cells committed to the T-lymphoid lineage are impaired by HIV in their differential pathway within the thymus (Bonyhadi et al, 1993). Infection of megakaryocytes could result in underproduction of platelets and possibly represents a major pathogenetic mechanism of HIV-related thrombocytopenia. Infection of monocytes and T-lymphocytes leads in vitro and probably also in vivo to deranged cytokine production. In the first stages of the disease, increased cytokine production, consequent to a chronic immune activation, is probably responsible for the myelodysplastic/hyperplastic alterations observed at the bone marrow level. In more advanced stages of the disease, the general decline in immune function, the consequent imbalance in cytokine production, and the increase in viral burden, may contribute to dysregulated haematopoiesis and peripheral blood cytopenias.