FEVER in children with acute leukemia may be a manifestation of the underlying disease or herald the onset of an infectious complication.1 Microorganisms that are generally considered to be of low virulence may become primary pathogens in patients whose innate defense mechanisms are altered by malignancy and immunosuppressive chemotherapy. The coliform bacteria, particularly Pseudomonas aeruginosa and Escherichia coli, and fungi of the Candida and Aspergillus species frequently occur in this role of opportunistic invader.2 Protozoa have been infrequently documented as significant pathogens in children with neoplasia.3 This report describes the clinical, morphologic, hematologic, and immunologic data of a child with acute toxoplasmosis acquired during a drug-induced remission of lymphoblastic leukemia. The diagnosis was made by microscopic identification of the nonencysted form of Toxoplasma gondii in aspirated bone marrow smears and confirmed by a rising fluorescent antibody inhibition titer. Recognition of the protozoan in bone marrow smears was