Background: Behcet's disease (BD) is a rare, chronic autoinflammatory disorder of unknown origin. Natural killer (NK) cells are one of the major immunoregulatory cell groups of the innate immune system, but their role in BD pathogenesis is not well documented.Objectives: We aimed to investigate the role of NK cell subsets and their cytokine secretion and cytotoxic activity in patients with BD.Patients and methods: The study group consisted of BD patients who had only mucocutaneous involvement, and they were compared with healthy subjects. BD patients were divided into two groups according to their frequencies of oral ulcerations. NK cell cytotoxicity was determined using CD107a expression and a CFSE-based cytotoxicity test. Expression of NK cell receptors and surface markers and the intracellular IL-5, IL-10, IL-17, and IFN- levels in CD16(+) NK cells were assessed by flow cytometry.Results: Although the cytokine secretion pattern was different, no difference was obtained in cytotoxic activity, expression of activatory receptors, or degranulation of NK cells.Conclusion: Increases in NK1/NK2 ratio and CD16(+)IFN-(+) NK1 cells might support the idea of a biased IFN- dominant immune response in the mucocutaneous involvement of BD pathogenesis. Although the cytokine secretion pattern was different, no difference was obtained in cytotoxic activity, expression of activatory receptors, or degranulation of NK cells.