Natural killer cell receptor (NKR)-expressing cells have cytolytic activity against leukemic cells, and solid tumor cells escape from T-cell recognition because of the low expression levels of class I HLA molecules in both allogeneic and autologous settings. This characteristic feature of NK cell recognition of target cells in contrast with that of T-cells provides a strategy to overcome tolerance in the tumor-bearing host. Furthermore, inhibitory NKR-expressing cells may have cytolytic activity and immunoregulatory functions. Several methods can be used to expand NKR-expressing cells for adoptive immunotherapy for leukemia and other malignant diseases. We review recent developments in the biology and clinical application of NKR-expressing cells, such as NK cells, lymphokine-activated killer cells, cytokine-induced killer cells, NKT cells, and other NKR-expressing cells.