A 20-yr-old man with bulky mediastinal and retroperitoneal tumour masses identified as myeloblastoma is described. After a partial remission was induced by aggressive chemotherapy, mediastinal irradiation and retroperitoneal tumour resection, the patient received an allogeneic marrow graft from his HLA-identical sister. The conditioning regimen consisted of high-dose busulfan and cyclophosphamide. The patient has a well-controlled secondary chronic graft-versus-host disease. He is in unmaintained complete remission and in good general condition at 20 months post-transplantation.