We describe a case of natural killer (NK) cell lymphoma/leukemia with only an interstitial deletion in the short arm of chromosome 12 as the primary event. Fluorescence in situ hybridization revealed that the ETV6 locus (12p13) and subtelomeric sequences are not deleted in the process. The p27/kip1 locus (12p12-13), a candidate tumor suppressor gene, was deleted on the abnormal chromosome. Sequence analysis detected an adenine nucleotide deletion in the third codon of exon 1 leading to frameshift and premature termination at codon 41 of the retained copy of p27/kip1. To the best of our knowledge, this is the first report in literature on a NK cell lymphoma/leukemia with complete loss of p27/kip1.