The putative natural killer (NK) cell lymphomas/leukaemias are a group of recently characterized haematolymphoid malignancies sharing an immunophenotype of CD3/Leu4- CD3epsilon+ CD56+, and a genotype of germline T-cell receptor genes. They frequently present in extranodal sites and exhibit a highly aggressive clinical course. Information on the cytogenetic or molecular events leading to the tumourigenesis in this group of tumours is very scarce. In this study we analysed the cytogenetic findings of seven patients with CD56-positive putative NK cell lymphoma/leukaemia. Three cases, including one nasal, one extranasal and one leukaemic form, showed a common region of deletion at 6q21-q25, suggesting that this may be a nonrandom chromosomal aberration.