A 57-year-old female was admitted to Uji hospital for the further evaluation of nodular shadow on her right lung. During the period of admission, she developed cervical lymph node swelling. She was diagnosed as having malignant lymphoma (diffuse, small cleaved cell) by lymph node biopsy. She received combined chemotherapy and obtained partial remission for seven months until she developed fever and pancytopenia. Laboratory data showed increased number of large granular lymphocytes (LGLs) in blood. Bone marrow revealed increased number of LGLs with hemophagocytosis by macrophage. Surface marker analysis revealed LGLs were positive for CD2 CD16, and CD56 and negative for CD3, CD4, CD8, and CD20. T-cell receptor genes beta and gamma were in germ line configuration. Analysis of Epstein-Barr virus genome using termini probe indicated a monoclonal proliferation of LGLs. Reexamination of the biopsy specimen of lymph node revealed LGLs which was negative for CD3 and CD20. The patient was diagnosed as a leukemic phase of natural killer (NK) cell lymphoma complicated with hemophagocytic syndrome (HPS). Serum levels of interferon-gamma, macrophage colony-stimulating factor, granulocyte colony-stimulating factor, and interleukin-6 increased, which might be related to HPS.