We describe a patient who had aggressive natural killer cell leukemia with profound hemophagocytosis. This combination must be underscored as one of several hemophagocytic syndromes. Activated phagocytes in the bone marrow appeared morphologically normal and could possibly be proliferating in response to some cytokine(s) such as interferon-gamma produced by leukemic cells, whose serum level was found to be extremely elevated in this case.