A variety of haematologic abnormalities are associated with infection by HIV, the human retrovirus that is the primary aetiologic agent of the acquired immunodeficiency syndrome (AIDS). We have reviewed the haematologic findings in well-characterized cohorts of patients with AIDS, AIDS-related complex (ARC) and asymptomatic homosexual men at risk for this retrovirus. Anaemia, granulocytopenia and thrombocytopenia were found in increasing prevalence according to the severity of clinical disease associated with retroviral infection. Bone marrow aspirations and biopsies revealed frequent hypercellularity, dysplasia, plasmacytosis and lymphoid infiltrates. These marrow morphologic findings were strongly associated with anaemia and granulocytopenia. Review of transfusion records of patients with HIV antibodies revealed a 21% prevalence of a positive direct antiglobulin test. The pathophysiology of the observed haematologic abnormalities may involve direct retroviral infection of bone marrow progenitors, abnormal regulation of haematopoiesis and/or autoimmune phenomena.