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Human natural killer (NK) cells are considered professional cytotoxic cells that are integrated into the effector branch of innate immunity during antiviral and antitumoral responses. The purpose of this study was to examine the peripheral distribution and expression of NK cell activation receptors from the fresh peripheral blood mononuclear cells of 30 breast cancer patients prior to any form of treatment (including surgery, chemotherapy, and radiotherapy), 10 benign breast pathology patients, and 24 control individuals. CD3 - CD56 dim CD16 bright NK cells (CD56 dim NK) and CD3 - CD56 bright CD16 dim/- NK cells (CD56 bright NK) were identified using flow cytometry. The circulating counts of CD56 dim and CD56 bright NK cells were not significantly different between the groups evaluated, nor were the counts of other leukocyte subsets between the breast cancer patients and benign breast pathology patients. However, in CD56 dim NK cells, NKp44 expression was higher in breast cancer patients (P = .0302), whereas NKp30 (P = .0005), NKp46 (P = .0298), and NKG2D (P = .0005) expression was lower with respect to healthy donors. In CD56 bright NK cells, NKp30 (P = .0007), NKp46 (P = .0012), and NKG2D (P = .0069) expression was lower in breast cancer patients compared with control group. Only NKG2D in CD56 bright NK cells (P = .0208) and CD56 dim NK cells (P = .0439) showed difference between benign breast pathology and breast cancer patients. Collectively, the current study showed phenotypic alterations in activation receptors on CD56 dim and CD56 bright NK cells, suggesting that breast cancer patients have decreased NK cell cytotoxicity., (Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.)