Two patients with high-risk acute myeloid leukemia (AML) whose bone marrow aspirates showed more than 25% blasts between 2 and 4 weeks after the first induction chemotherapy immediately received modified conditioning therapy with intravenous busulfan at 50% of the usual dose and fludarabine, before hematologic recovery occurred. Unmanipulated G-CSF mobilized peripheral blood stem cells from an HLA-identical sibling donor were transfused and haematopoietic recovery was achieved in both recipients. Both of them are in continuing hematological remission with full donor chimerism 12 and 22 months after transplantation. Early treatment intensification with allogeneic cell therapy during marrow aplasia might cure high-risk AML patients who are unlikely to achieve remission with conventional chemotherapy protocols.