We describe a 54-year-old man with CD2, CD7, and CD56-positive but CD3, CD4, and CD8-negative aggressive NK cell lymphoma/leukemia. Chromosome analysis of the peripheral blood cells showed clonal aberration consisting of 46,XY,dup(3)(p21p25),der(3)t(1;3)(q12;p25),del(5)(q13q22), del(6)(q13),del(13)(q12q14). The peripheral blood lymphoma cells contained clonal EBV-DNA by Southern blot analysis. The disease was refractory to chemotherapy and the clinical course was aggressive and rapid. Deletion 6q and deletion 13q have been frequently reported in NK cell lymphoma/leukemia. Thus, some tumor suppressor genes involving NK cell malignancies may be present in these regions.