Significant engraftment variability occurs among patients following nonmyeloablative hematopoietic cell transplantation. We analyzed the impact of multiple factors on donor myeloid and T-cell engraftment in 36 patients with metastatic tumors undergoing cyclophosphamide/fludarabine-based conditioning. Higher CD34(+) doses facilitated donor myeloid engraftment, while prior chemotherapy exposure facilitated both donor myeloid and T-cell engraftment. At day 30, median donor T-cell and myeloid chimerism was 98% and 76%, respectively, in those patients with prior chemotherapy versus 88% (P =.008) and 26% (P <.0001) in chemotherapy-naive patients. Donor myeloid chimerism at day 45 was predicted by prior chemotherapy exposure and the log(10) of the CD34(+) dose (adjusted coefficient of determination [R(2)] =.47; P <.0001), while chemotherapy alone impacted donor T-cell engraftment. Patients with prior chemotherapy were more likely to develop acute grades II to IV graft-versus-host disease (GVHD; 8/18) compared with chemotherapy-naive patients (2/18; P =.031). Thus, tailoring the intensity of nonmyeloablative conditioning based on prior chemotherapy exposure is an important consideration in trial design.