Aggressive NK-cell leukemia is a rare malignancy mostly seen in younger Asians with a rapid clinical course and poor prognosis. Here, we describe a 69 years old Caucasian woman presenting with massive leukemization of neoplastic NK-cells. The cells were abnormal in morphology and surface marker expression and this clearly distinguished them from their normal counterpart. They were large and variable in shapes with irregular folding of the nuclei. By flow cytometry, their light scatter characteristics resembled normal monocytes. They showed bright expression of CD56 and CD2 but markedly decreased expression of CD7. They also expressed CD25. The patient presented with general malaise, including high fever, abdominal pain, signs and haemophagocytosis, and she quickly deteriorated and died 11 days after hospitalization. The origin of the leukemic cells of aggressive NK-cell leukemia is most likely the relatively scarce population of CD56bright NK-cells, primarily residing lymph nodes and tonsils. The immunophenotype of the case presented here support this, adding CD25 expression which is not earlier addressed in this entity.Electronic supplementary materialThe online version of this article (doi:10.1186/s40064-015-1553-y) contains supplementary material, which is available to authorized users.