Acquired hyperthyroidism is most commonly autoimmune in etiology. In the setting of allogeneic bone marrow transplantation (BMT), the use of radiotherapy (total body irradiation) as part of the regimen prior to BMT is known to cause endocrine dysfunction, especially hypopituitarism and hypothyroidism, but hyperthyroidism is rare. The authors report this unusual and late complication in a young boy after BMT for relapsed childhood lymphoblastic leukemia and discuss the possible etiologies.