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We report a comprehensive study of a case of aggressive natural killer cell lymphoma/leukemia, which is characterized by young male predominance, rapidly progressive clinical course, and presence of lymphadenopathy, hepatosplenomegaly, and bone marrow involvement. The leukemic phase is frequently preceded by pancytopenia. The diagnostic clues are the detection of cytoplasmic granules in tumor cells on Wright-Giemsa-stained tissue imprints or smears and a selective loss of T-cell antigens. Immunophenotyping is decisive in making the final diagnosis by showing positive natural killer cell markers (CD16, CD56, and/or CD57), CD2, CDllc, and la, but negative CD3, T-cell receptor heterodimers, terminal deoxynucleotidyl transferase, and B-cell markers. Genotyping always shows germline configuration in both immunoglobulin and T-cell receptor genes. The unique feature in this case is its presentation as a testicular lymphoma, which has not been previously reported. Polymerase chain reaction was performed in this case but failed to detect human T-cell leukemia virus type I/II provirus. It is important to recognize this new entity as it is a highly aggressive disease with a rapidly progressive clinical course and fails to respond to any chemotherapeutic regimen available.