The frequency and clinicopathological significance of the expression of natural killer cell receptors (NKRs) in T-cell malignancies remain undefined. A 71-year-old man presented with leukocytosis, generalized lymphoadenopathy, and hepatosplenomegaly. Bone marrow and lymph node biopsies showed a T-cell lymphoproliferative disease expressing NKRs (CD2(+), CD3(+), CD4(+), CD5(+), CD7(+), CD8(-), CD56(-), CD94(+), CD158a(+), CD158b(+), CD161(-), p70(-), TCRalphabeta(1), TCRgammadelta(2), TIA-1(-)). An abnormal clone, 46,Y,add(X)(p14),der(1)t(1;6)(p33;p21),t(7;12)(p10;q10), was found on conventional karyotyping. Comparative genomic hybridization confirmed these findings, and showed a deletion of 12p that was not apparent on karyotyping. Clinically, the disease remained indolent and responded transiently to purine analogs but not to intensive chemotherapy. Peripheral T-cell lymphoproliferative disease of CD4(+)alphabeta(1)NKR(+) phenotype is hitherto undescribed. The issues of whether this case was derived from transformation of a rare T-cell subtype or represented aberrant T-cell expression of NK-cell antigens, and the clinicopathologic significance of these T-cell neoplasms warrant further studies.