Aggressive natural killer cell leukemia (ANKL) is a rare malignant disease of NK cells that has a median survival of less than 2 months and a strong association with the Epstein-Barr virus. Herein, we report three Japanese cases of the disease. A 21-year male patient, a 31-year female patient, and a 76-year female patient presented with high fever, lymphadenopathy, hepatosplenomegaly, and severe liver damage. All three cases had granular lymphocytes in both peripheral blood and bone marrow. The phenotype of these cells was CD2(+)CD3(-)CD56(+)HLA-DR(+). All cases had a high copy number of serum Epstein-Barr virus DNA in the peripheral blood and were diagnosed with ANKL. Case 1 and Case 2 were treated with chemotherapy, but suffered from gross intestinal bleeding or massive bleeding in the cerebellum, resulting in death. Although not treated with chemotherapy, Case 3 also suffered gross bleeding from an atypical duodenal ulcer and died from hemorrhagic shock 15 days after admission. There have been no previous reports of such acute lethal hemorrhagic complications with ANKL. The present cases suggest that patients with ANKL need a sufficient supply of coagulation factors, and that chemotherapy for this disease should be carefully designed with promising agents. [J Clin Exp Hematopathol 52(2) : 101-106, 2012].