Three marrow transplant recipients with hematologic malignancies (two AML, one myelodysplastic syndrome) experienced prolonged pancytopenia after allogeneic BMT following conditioning with non-TBI regimens containing high-dose busulfan and cyclophosphamide (Bu/CY), despite the use of G-CSF. Early recovery of host-derived hematopoiesis ensued. Although neutrophil counts in these patients exceeded 500 x 10(6)/l by day 30 after transplant, these cells were of host origin. This early recovery of host-derived hematopoiesis has been observed rarely among patients conditioned with TBI-based regimens. When patients conditioned with Bu/CY show delayed hematologic recovery, mixed chimerism should be considered even in the presence of normal neutrophil recovery.