Blastic natural killer cell leukemia/lymphoma (blastic NKL/L) is characterized by blastic morphology and a distinctive immunophenotype combining blastic features and cytologically resembling acute myeloid or lymphoid leukemia. The clinical, pathologic, and cytogenetic features of blastic NKL/L have not yet been systematically identified. We report herein a case of blastic NKL/L with skin lesion, adenopathy, and systemic lymphoadenopathy. The identified tumor cells were positive for CD4 and CD56, and negative for T-cell, B-cell, and myeloid markers. T-cell receptor beta, gamma, delta, and immunoglobulin heavy chain genes in the bone marrow cells showed germ-line configurations. Southern blot analysis with a terminal probe did not reveal any Epstein-Barr virus infection. Although patients diagnosed as blastic NKL/L have generally shown chemotherapy resistance and poor prognosis, our patient was treated with a combined chemotherapy, which is also used for acute lymphoblastic leukemia, and has maintained complete remission (CR) for more than 13 months. In addition to clinical investigations, we thoroughly analyzed his karyotype by using a combination of G-banding and a new technique, spectral karyotyping. The karyotype was described as 45, XY, der(1)t(1;20)(p32;q11.2), der(6) (1pter-->1p32:: 6p21.1-->6q13:: 7q11.2-->7qter), der(7) t(7;20)(q11.2;q11.2), t(13;14)(q14;q32), der(13)t(6;13) (p21.1; q14), -20.