As outcomes of hematopoietic stem cell transplantation have improved over time, disease relapse has emerged as the most significant cause of treatment failure. Cellular therapy represents an alternative therapeutic approach, not only to treat or prevent disease relapse after hematopoietic stem cell transplantation, but also conceivably to help patients achieve remission prior to transplantation or as consolidation therapy for high-risk patients with hematologic malignancies. Of the many cellular therapies available, infusion of NK cells may be the most promising approach against malignant or virally infected cells owing to strong innate activity of NK cells in vitro and in vivo. Only limited clinical data exists mostly from feasability studies of inadequate size to demonstrate clinical benefit. Here, we discuss the current status of clinical investigation using NK cell therapy for patients with hematologic malignancies undergoing hematopoietic stem cell transplantation.