We report two patients with leukaemic proliferations of large granular lymphocytes. The immunophenotype study showed that the leukaemic cells were positive for CD2, CD38, CD56 and anti-HLA-DR monoclonal antibodies and negative for other T-cell (CD3, CD4, CD8) and B-cell markers (CD19, CD20 and surface immunoglobulins). The clinical course was acute and a diagnosis of aggressive natural killer cell leukaemia/lymphoma was made. No clonal rearrangements of either C beta T-cell receptor or JH immunoglobulin genes were found. Functional studies done in one patient demonstrated non-restricted cytotoxic activity after activation with IL-2. Lethal midline granuloma had been previously diagnosed in both patients. A possible relationship between this entity and the natural killer cell leukaemia is discussed.