Only a few blastic natural killer (NK) cell leukemias and lymphomas have been reported. As such, the clinicopathologic spectrum of this disease is incompletely understood. We report 7 cases of blastic NK cell lymphoma/leukemia. All patients were men, 5 white and 2 Arab American. All cases exhibited blastic morphologic features and were CD3- and CD56+ with germline T-cell receptor genes. Five cases were CD4+ and involved the skin. Both CD4- cases never involved the skin. Other markers of mature NK cells such as CD16, CD57, and TIA-1 were expressed infrequently. Three cases were CD33+. One CD33+ case had a clonal rearrangement of the immunoglobulin heavy chain gene. Skin and lymph nodes were involved most often, with frequent evolution to a leukemic phase. Initial responses to therapy were achieved in most patients, but the tumors invariably recurred.