Natural killer (NK) cells are lymphocytes with large granular lymphocyte morphology, CD3-CD56+ phenotype, non-MHC-restricted cytotoxicity, and germ-line configuration T-cell receptor genes. Two types of lymphomas originating from NK cells have been described; blastic NK-cell lymphoma, and nasal-type NK-cell lymphoma. Because recent reports indicate that blastic NK-cell lymphoma originates from the precursors of plasmacytoid dendritic cells, I will focus mainly on nasal-type NK-cell lymphoma, and discuss its pathogenesis, diagnostic problems, treatment strategy, and outcome. Nasal-type NK-cell lymphoma develops mostly in the nasal cavity and rarely in other sites, such as the skin and intestinal tract. Epstein-Barr virus (EBV) is found in lymphoma cells of almost all the patients, and is considered to be the etiologic agent. Indeed, EBV easily infects NK cells in the absence of CD21 antigen, or EBV receptor, on the surface of NK cells. Further, various types of oncogenes and suppressor oncogenes are found to be involved in its pathogenesis. Based on the data obtained from paraffin-embedded specimens, it is difficult to determine whether the lymphoma cells are of T-cell or NK-cell lineage, because immunohistochemical staining of cytoplasmic CD3 is positive both in T and NK cells, and CD56 is positive in a part of T cells. The presence of CD5 antigen indicates T-cell lineage. When the disease is limited, radiation therapy is effective, but not satisfactory. A new trial to use simultaneously both radiation and chemotherapy has started in Japan. In advanced stages, a combination chemotherapy including L-asparaginase seems to be promising, and high-dose chemotherapy with autologous or allogeneic stem cell support is under investigation. A recent report described the expression of short-length P-glycoprotein (P-gp), but not full-length P-gp in NK cells, and this mini-P-gp is unable to extrude daunorubicin. These findings may change the treatment strategy. Finally, I will present the results on interim analysis of 166 cases of nasal-type NK-cell lymphoma collected in Japan between 1994 and 1998.