The aim of this study was to investigate the effects of natural killer (NK) cells on transplant outcomes in patients receiving G-CSF-mobilized PBSC grafts and G-CSF-primed BM grafts from HLA-haploidentical donors. Forty-one haploidentical allogeneic hematopoietic SCT patients were analyzed according to the NK cell concentration in relation to acute GVHD (aGVHD), chronic GVHD (cGVHD), TRM and leukemia-free survival. The patients receiving a higher dose of CD56(bright) NK cells (>1.9 x 10(6)/kg) showed a higher incidence of grades II-IV aGVHD (hazard risk (HR), 2.872; P=0.022) and cGVHD (HR, 2.884; P=0.039). A higher CD56(dim)/CD56(bri) NK cell ratio (>8.0) was correlated with a decreased risk of III-IV aGVHD (HR, 0.290; P=0.065) and TRM (HR, 0.072; P=0.012), thereby increasing the rate of leukemia-free survival (HR, 0.174; P=0.007) after haploidentical transplantation without in vitro T-cell depletion. Our results suggest that a high allograft CD56(dim)/CD56(bright) NK cell ratio (>8.0) plays an important role in improving transplant outcomes. A higher dose of CD56(bright) NK cells might be a predictor for a higher incidence of GVHD.