A bone marrow biopsy was performed on four patients with acquired immune deficiency syndrome (AIDS) for a long-running course fever of unknown origin associated with a recent pancytopenia. In the four cases, striking histological similarities, such as interstitial edema, foci of necrosis and only few scattered or clustered histiocytes, were found. Near or in the foci of necrosis, free forms, and pseudocysts of Toxoplasma gondii were observed not only in the cytoplasm of macrophages and of some granulocytes, but also within megakaryocytes. No sign of other parasitic, bacterial, or fungus infection has been found. The diagnosis was confirmed by immunohistochemistry in the four cases and ultrastructural examination in one case. This case study stresses the importance of bone marrow histological changes for the diagnosis of severe toxoplasmosis in AIDS patients and particularly the localization of T gondii within the cytoplasm of megakaryocytes.