Natural killer cell leukemia is a part of large granular Iymphocyte(LGL) leukemia, which has more rapidly progressive clinical course than other types of LGL leukemia. We experienced a 60 year-old female who had had hepatosplenomegaly and a fever for one month. On CBC, white blood cell count was 19.5×10⁹/L, hemoglobin 95 g/L, platelet count 41×10⁹/L. The LGLs made up 58% of peripheral leukocytes. Most of them were 15-20μm in diameter and had moderate amount of the cytoplasm with clumped nuclear chromatin. Several azurophilic granules were noted in their cytoplasm.
These cells were positive for acid phosphatase(AP) and negative for Periodic Acid Schiff(PAS), Sudan Black B(SBB), nonspecific esterase(NSE), tartrate-treated acid
phosphatase(TTAP) and terminal deoxynucleotidyl transferase(TdT).The immunophenotype of the cells was CD2+, CD3-, CD8-, CD16+, CD56+ that are characteristic of natural killer cells. The patient showed acute elevation in Iymphocyte
count up to 31,600/μL and expired on the 11th hospital day.

Keywords: Leukemia, NK cell, Large granular Iymphocyte,