Peripheral blood cytotoxic lymphocytes, including gamma delta+ T cells, natural killer (NK) cells, and CD56+ T lymphocytes were investigated by using 2-color flow cytometry in patients with Behçet's disease. The percentage of circulating gamma delta+ T cells among total lymphocytes or among total T cells was significantly increased in patients with Behçet's disease, compared to healthy controls. An increase in circulating gamma delta+ T cells was related to mucocutaneous lesions, but not the HLA-B51. Among gamma delta+ T cells, the number of CD8+ or CD56+ subset was significantly increased in patients with Behçet's disease. Moreover, significant increases of peripheral blood NK cells (CD16+CD56+CD3-, CD2+CD3-, CD8dim+CD3-) and CD56+ T lymphocytes were also observed. Our data suggest that increased circulating gamma delta+ T cells and NK cells may play an important role in the pathogenesis of Behçet's disease.