Background <p>Immunotherapy offers a promising approach to prevent relapse and transplant complications. Natural killer (NK) cells play a crucial role in the immune system’s initial defense against infections and malignant cells. Activation of NK cells with interleukin-15 enhances anti-leukemic immune responses. We conducted this clinical trial to evaluate the safety and efficacy of CD56 + cell infusion in increasing survival and reducing relapse in patients with acute myeloid leukemia.</p> Methods <p>Three escalating doses of 1 × 10⁶, 3 × 10⁶, and 5 × 10⁶ interleukin-15-activated CD56 + cells/kg derived from peripheral blood of haploidentical donors were injected on days + 7, +14, and + 21 following transplantation. Patients were monitored for adverse events. To evaluate efficacy, bone marrow samples were taken on days + 30 and + 90. On day + 40, the population of NK cells and its correlation with patients’ clinical status were evaluated. Besides, immune reconstitution was examined on day + 40. Patients were followed up for one year.</p> Results <p>The CD56 + cells infusion was safe and not associated with any complications. While survival rates were not statistically different between the intervention and control groups, a trend toward improved survival was observed in the intervention group. Patients with complete remission showed a higher percentage of NK cells on day + 40 than patients who experienced graft rejection, relapse, or death.</p> Conclusions <p>In this study, we introduce a feasible method for activating CD56 + cells. Although the survival rates of patients in the intervention and control groups did not differ statistically, this study showed that a higher percentage of NK cells on day + 40 is associated with better survival and fewer complications following transplantation.</p> Trial registration <p>This study was registered at the Iranian Registry of Clinical Trials under code IRCT20230801058996N2 on 2023-09-10.</p>