The prognosis of patients with advanced-stage extranodal natural killer/T cell lymphoma, nasal type (ENKL) has been generally poor, and several anecdotal reports have suggested the role of allogeneic hematopoietic stem cell transplantation (HSCT). A potential advantage of allogeneic HSCT may be the graft-versus-lymphoma (GVL) effect. The susceptibility to the GVL effect, however, has been shown to vary according to histologic subtypes, and it has been hardly documented yet whether ENKL is susceptible to the GVL effect. Here we report a patient with advanced-stage ENKL who underwent allogeneic HSCT from an HLA one-allele mismatched related donor, whose clinical course after HSCT suggested the potent GVL effect against ENKL. A 43-year-old female underwent allogeneic HSCT for advanced-stage, chemorefractory ENKL, and achieved complete response. In 4 months after the transplantation, however, the ENKL relapsed in multiple sites. These lesions markedly responded to the discontinuation of immunosuppressive agents and disappeared. Except for a temporal exacerbation of bronchiolitis obliterans organizing pneumonia, she has been free from disease for more than a year without other treatments against lymphoma. The clinical course of the current patient suggests the potent GVL effect against ENKL. Allogeneic HSCT, including that with reduced-intensity regimens, is a promising treatment option for high-risk ENKL. Am. J. Hematol. 82:937–939, 2007. © 2007 Wiley-Liss, Inc.