Natural killer cell malignancy is a rare and aggressive lymphoid neoplasm encompassing extra-nodal NK/T-cell lymphoma, nasal-type (ENKLN) and aggressive NK-cell lymphoma/leukemia (ANKL). A case of cutaneous ENKLN and a case of ANKL in Thai patients are reported Both patients developed hemophagocytic syndrome and shortly succumbed to death. The cells in cutaneous ENKLN are small to medium in size with minimal cytoplasm, round nuclei, irregular nuclear membrane, andfine chromatin with inconspicuous nucleoli. While that of ANKL are medium to large-sized mononuclear cells with moderate cytoplasm. Their nuclei are elongated to embryo-like with irregularly thickened nuclear membrane, fine chromatin, and small to occasional prominent nucleolus. Ancillary techniques studied on paraffin embedded tissues of both cases demonstrated that the neoplastic cells exhibit cytoplasmic CD3+, CD56+ and cytotoxic granules + by immunohistochemistry, absence of T cell receptor gene rearrangement by PCR, and presence of Epstein-Barr virus mRNA (EBER) transcripts by in situ hybridization. The authors reviewed the literature on natural killer cell neoplasm and compared the clinical characteristics, natural history, and association of Epstein-Barr virus infection with hemophagocytic syndrome.