We analyzed the kinetics of donor engraftment among various peripheral blood cell subpopulations and their relationship to outcomes among 120 patients with hematologic malignancies given hematopoietic cell transplantation (HCT) after nonmyeloablative conditioning consisting of 2 Gy total body irradiation (TBI) with or without added fludarabine. While patients rapidly developed high degrees of donor engraftment, most remained mixed donor/host chimeras for up to 180 days after HCT. Patients given preceding chemotherapies and those given granulocyte colony-stimulating factor-mobilized peripheral blood mononuclear cell (G-PBMC) grafts had the highest degrees of donor chimerism. Low donor T-cell (P = . 003) and natural killer (NK) cell (P = VSports手机版. 004) chimerism levels on day 14 were associated with increased probabilities of graft rejection. High T-cell chimerism on day 28 was associated with an increased probability of acute graft-versus-host disease (GVHD) (P = . 02). Of 93 patients with measurable malignant disease at transplantation, 41 achieved complete remissions a median of 199 days after HCT; 19 of the 41 were mixed T-cell chimeras when complete remissions were achieved. Earlier establishment of donor NK-cell chimerism was associated with improved progression-free survival (P = . 02). Measuring the levels of peripheral blood cell subset donor chimerisms provided useful information on HCT outcomes and might allow early therapeutic interventions to prevent graft rejection or disease progression. .