Although opportunistic infections after allogeneic bone marrow transplantation are common, cerebral toxoplasmosis has only been reported once. We report a case of lethal cerebral toxoplasmosis in a patient with acute leukemia who had been submitted to intensive cytotoxic therapy and allogeneic marrow rescue. The clinical course was characterized by an unusually sudden neurological onset. It was remarkable that, in this case, toxoplasma serology remained repeatedly negative. The autopsy revealed that the infection also involved the myocardium. Serological data and pathology findings are presented.