Aggressive NK-cell leukemia (ANKL) has a fulminant clinical course with a poor prognosis. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is currently the only curative treatment. Using the Japanese transplant registry data, the outcomes of 59 ANKL patients who underwent first allo-HSCT were analyzed. Twenty-nine patients received stem cells from cord blood (CB), 18 from peripheral blood, and 12 from bone marrow. At the time of transplant 21 patients had complete response (CR), and 7 partial response (PR), but 31 without response. The 1-year and 5-year overall survival (OS) were 33.9% and 27.3%, respectively. The 1-year cumulative incidences of relapse or progression was 55.5%, and that of non-relapse mortality was 12.1%. The OS was significantly better for patients with CR or PR at the time of allo-HSCT (P = 0.046), which was equivalent to that for patients who experienced primary induction failure at the time of allo-HSCT but achieved CR afterwards (40.6% versus 32.0% at 5 years; P = 0.95). Patients receiving CB had a significantly better OS than those receiving stem cells from others (37.3% versus 16.2% at 5 years; P = 0.04). Patients achieving event-free survival at 12 months after allo-HSCT had good outcomes with 5-year OS of 85.2%.