10034 Background: In the setting of hematopoietic stem cell transplantation (HSCT), donor natural killer (NK) cells exhibit potent anti-leukemic effects without causing graft-versus-host disease. We hypothesized that the transplantation of purified haploidentical NK cells may be a safe and effective form of consolidation therapy that will reduce the risk of relapse among children with acute myeloid leukemia (AML) who are not treated with HSCT. In this pilot study, we assessed the safety, feasibility, and engraftment of NK cell infusions in 10 patients with AML in first remission.Methods: Patients received cyclophosphamide, 60 mg/kg on day -7; fludarabine, 25 mg/m2/day on days -6 through -2; and IL-2, 1 million units/m2, every other day for 6 doses starting on day -1. On day -1, the donor underwent apheresis and the product was purified for CD56+ cells by a two-step procedure. The entire purified product was infused on day 0.Results: The 10 patients had a median age of 2.5 years (range, 8 months to 21 years) and a median leukocyte count of 62 x 109/L (range, 4 to 487) at diagnosis. Leukemic cell genetic abnormalities included CBFβ-MYH11in 4 cases, RBM15-MKL1in 2 cases, MLL-ENL and MLL-AF9 in 1 case each; 2 cases had no detectable abnormalities. Patients received a median of 29 × 106/kg NK cells (range, 5 to 81 × 106/kg). All patients had detectable donor NK cells at one or more time points: donor NK cell chimerism ranged from 0% to 30% during the first 4 weeks after the infusions and was greater than 1% in 9 cases at week 1, 4 cases at week 2, 5 cases at week 3, and 3 cases at week 4. One patient had prolonged NK engraftment (189 days), but no non-hematological toxicity. Grade 3–4 non-hematological toxicity was limited to one respiratory viral infection and one episode of febrile neutropenia. Median length of hospitalization was 2 days (range, 0–3) and median time to neutrophil recovery was 12 days (range, 9–56). With a median follow-up time of 637 days, all patients remain in remission. Conclusions: Haploidentical NK cells can be safely administered to AML patients who are in remission. All patients demonstrated temporary engraftment, which may have antileukemic effects. We have recently opened a new trial to evaluate the efficacy of NK cell therapy in children in first remission of AML. No significant financial relationships to disclose.