Post transplant infusion of donor-type natural killer (NK) cells has been shown to have an anti-leukemia-enhancing effect without evoking GVHD in murine hematopoietic cell transplantation (HCT) models. Here, we tested 14 patients (age, 23-65 years), 12 with acute leukemia and 2 with myelodysplastic syndrome, who underwent HLA-mismatched HCT and subsequently received donor NK cell infusions. Cell donors (age, 16-51 years), comprising seven siblings, five offspring, and two mothers of the patients, underwent growth factor-mobilized leukapheresis for 3-5 days. Cells collected on the first 2-4 days were used for HCT, whereas those collected on the last day were CD34 selected by magnetic-activated cell sorting (median, 2.22 x 10(6) cells/kg; range, 0.29-5.66). Donor NK cells were generated from the CD34(+) cells by ex vivo cell culture over a 6-week period (median, 9.28 x 10(6) cells/kg; range, 0.33-24.50; CD122/CD56(+) 64%; CD3(+) 1.0%; and viability 88%). There were no signs of acute toxicity in patients infused with these cells 6-7 weeks post transplant. Overall, one and five patients developed acute and chronic GVHD during post transplant period, respectively. These results showed that clinical-grade donor NK cell production from CD34(+) cells is feasible.