A rare form of putative natural killer (NK) cell lymphoma called blastic NK cell lymphoma appears to be clinicopathologically distinctive in showing a homogenous lymphoblast, variable expression of CD2, CD4, CD56 and TdT, negative for surface CD3, T-cell receptor antigen, CD16, CD34 and lack of association with Epstein-Barr virus (EBV). We report two patients with blastic NK cell lymphoma and describe the interesting clinical studies. The patients presented with cutaneous plaques. Both patients had adenopathy, and one had marrow involvement at presentation. Unlike in many NK and NK-like T-cell disorders, azurophilic cytoplasmic granules were absent. They expressed intermediate density CD45. In addition, the cells were positive for HLA-DR, CD2, CD4, CD56 and TdT, and negative for EBV transcripts. In spite of the advanced clinical stage, complete remission was achieved by conventional chemotherapy. After interleukin 2 expansion of tumour-infiltrating bone marrow and lymph node cells from the patients, cytotoxic T-cell lines with rearranged T-cell receptor genes were established. They showed specific killing activity against autologous tumour cells in an MHC-restricted fashion, with possible implications for treatment. In addition, upon cessation of maintenance chemotherapy, one patient developed overt leukaemia with blasts expressing CD33 antigens, suggesting a continuous spectrum of blastic NK cell lymphoma to myeloid/NK cell precursor acute leukaemia.