We report the clinical course of five adult patients with chronic Chagas' disease (Cd) who underwent BMT. Two patients with non-Hodgkin's lymphoma and one with ALL received an ABMT. Allogeneic BMT was performed in two patients with AML and CML respectively. One donor had chronic Cd. Samples of peripheral blood for parasite investigation by the Strout method, blood culture, and immunological studies by indirect immunofluorescent assay, ELISA and indirect hemagglutination tests were performed weekly from the start of chemotherapy until day +60 for ABMT and during the period of immunosuppression for allogeneic BMT. No prophylaxis was given to any of these patients. In only one ABMT patient were trypomastigotes detected early by blood culture without symptoms of reactivation. Benznidazole as preemptive treatment was administered at 5-8 mg/kg/daily for 30 days. Parasitemia was rapidly cleared and at the end of therapy xenodiagnosis was negative. The other Cd patients showed no evidence of relapse of parasitemia or signs and symptoms of reactivation. In brief, evidence of Cd should be sought in all BMT patients coming from endemic areas because parasitemia and reactivation are potential complications during the period of neutropenia and immunosuppression. The strategy used for early detection and treatment of parasitemia and reactivation was safe and effective.