Blood and marrow specimens were evaluated from 12 patients with the acquired immune deficiency syndrome (AIDS). Ten patients were anemic, eight leukopenic, and three thrombocytopenic. Pancytopenia was present in two patients and subsequently developed in two others. Reticulocyte counts were not increased in the anemic patients. The most common peripheral blood abnormalities were a left shift in the granulocyte series, lymphopenia, atypical lymphocytes, and vacuolated monocytes. Marrow cellularity was increased in five patients and reduced in three. Marrow reticulin was increased in 10 patients; in three of these, marrow could not be obtained by aspiration. Plasma cells were increased in number in every marrow aspirate, and there was a left shift in the myeloid series in most. Aggregates of atypical lymphocytes or a diffuse increase in marrow lymphocytes occurred in seven patients. An increase in histiocytes was observed in seven marrow aspirates; in five of these, the histiocytes were phagocytizing red cells, white cells, and platelets. Necrosis was present in four marrow specimens. These hematologic abnormalities reflect, in part, the presence of systemic infection, inflammation, and the inanition associated with them. However, the high incidence of myelofibrosis, alterations in marrow cellularity, pancytopenia, and hematophagic histiocytosis indicates that the bone marrow is a target organ in AIDS.