Objective To investigate clinical characteristic and prognosis of aggressive natural killer cell leukemia (ANKL) with focal bone marrow lesions and hemophagocytic syndrome. Methods On 29 August 2016, a case of ANKL with focal bone marrow lesions and hemophagocytic syndrome who were admitted to Department of Hematology, Huai′an First People′s Hospital was included in this study. The patient was diagnosed based on clinical features and related examinations. The patient refused to receive chemotherapy. Dexamethasone 10 mg/d×12 d was given intravenously to control hemophagolysis; sodium foscarinate sodium chloride 2.4 g/time×2 times/d×4 d was given intravenously for antiviral treatment; cefoperazone sodium sulbactam sodium 1.5 g/time×3 times/d×3 d was given intravenously. The anti-infective effect was not good, and the replacement drug was teicoplanin 400 mg/time×2 times/d on the first day, and then 400 mg/d×7 d from the second day; imipenem sodium butyrate 1 g/time×3 times/d×3 d was given intravenously. At the same time, 5 U of the infusion red blood cell preparation, 2 U of the platelet preparation, and 325 mL of fresh plasma were administered. The laboratory test results, diagnosis, therapeutic effect and other clinical data of ANKL patients with focal bone marrow lesions and hemophagocytic syndrome were analyzed retrospectively, and the research progress on diagnosis and treatment of ANKL was discussed in combination with relevant literature. Results ① After admitted, patients′ abnormal index of blood routine examination showed that white blood cell count was 2.28×109/L, neutrophil count was 1.32×109/L, red blood cell count was 2.75×109/L, hemoglobin (Hb) level was 68 g/L, platelet count was 89×109/L, monocyte count was 0.42×109/L, and lymphocyte count was 0.52×109/L. Abnormal index of coagulation function test showed that international normalized ratio (INR) was 1.18, prothrombin activity (PTA) was 77%, activated partial prothrombin time (APTT) was 46.2 s, and other indicators were within the normal reference range. EB virus (EBV) quantitative test results were 8.74×103 copies/mL (normal reference value range <3.00×103 copies/mL). Cytomegalovirus (CMV) detection result was negative. Results of liver function tests showed that level of lactate dehydrogenase (LDH) was 293 U/L, and other indicators were within the normal reference range. Abdominal CT results showed splenomegaly. Ultrasound examination showed that the liver and spleen were large. The smear of the patient′s bone marrow (right anterior iliac) on 30 August 2016 showed a ratio of 4% of ANKL cells. Bone marrow cell immunophenotyping results showed 2 subpopulations, P1 with a ratio of 8.4% and P2 with a ratio of 3.8%. P1 antigen included myeloperoxidase (MPO) (0.1%), cCD3 (81.7%), CD2 (79.6%), CD5 (80.3%), CD7 (70.2%), CD56 (5.9%); P2 antigens included MPO (0.1%), CD2 (98.2%), CD7 (97.8%), CD56 (92.0%), human leukocyte antigen (HLA)-DR (98.4%). Thirty-three common leukemia fusion genes and 4 myeloid gene mutations were negative. TCR/IGH gene rearrangement was also negative. On 1 September 2016, the smear of the bone marrow (left posterior sputum) showed a ratio of ANKL cells (tissue cell-like) of 1.5%. No obvious abnormalities caught in bone biopsy results. On 5 September 2016, the results of smear examination of bone marrow (left anterior) showed that the proportion of ANKL cells was 70.5%, and result of peroxidase (POX) staining was negative with hematopoietic cells. Results of immunophenotypic analysis showed that proportion of 1 subpopulation cells was 50.1%, and the antigens included MPO (0.1%), CD2 (92.6%), CD7 (92.2%), CD56 (82.2%), HLA-DR (85.9%), CD38(85.1%). Karyotype analysis results show that 46XX, add(4)(q35), add(13)(q34)[1]/46, XX[19]. In addition, the patient repeatedly suffered from high fever with a heat peak of 39 ℃ for more than one week. ② The patient was diagnosed as ANKL with focal bone marrow lesions accompanied by hemophagocytic syndrome. ③After treatment, on 8 September 2016, Hb level, platelet count, and red blood cell count of the patient decreased progressively; on 12 September 2016, white blood cell count, bilirubin level, aminotransferase level, and LDH level increased progressively. On 15 September 2016, re-examination of fibrinogen level was 1.61 g/L, and this suggested abnormal coagulation function; on 13 September 2016, EBV quantitative test results were 8.74×103 copies/mL; multiple lymph nodes were enlarged; liver and spleen progressively enlarged. On 16 September 2016, the patient developed respiratory failure, and pulse oxygen decreased with cardiac dysfunction. The patient′s family gave up further treatment, and the patient died. Conclusion ANKL with focal bone marrow lesions and hemophagocytic syndrome has an invasive clinical course, and disease deteriorates in a short period of time with poor prognosis. Key words: Leukemia, large granular lymphocytic; Epstein-Barr virus infections; Diagnosis; Prognosis; Bone marrow with focal type