Behçet's disease is a systemic vasculitis of unknown etiology. Various immune abnormalities have previously been shown in Behçet's disease. We investigated T lymphocyte subsets associated with cytotoxic activity and natural killer (NK) cells by flow cytometry in 37 patients with Behçet's disease, 38 healthy controls, and 17 diseased control patients. Compared to the healthy controls, CD4+CD16+ and CD4+CD56+ subsets were found to be higher in the Behçet's disease group as well as in the disease control group (CD4+CD16+: BD = 5 +/- 3, DC = 14 +/- 14, HC = 3 +/- 2, P = 0.001; CD4+CD56+: BD = 11 +/- 5, DC = 18 +/- 17, HC = 8 +/- 6, P = 0.01). CD8+CD16+ and CD8+CD56+ T cell subsets were at normal levels in Behçet's disease but found to be elevated in disease controls. Similarly, NK cells (CD16+CD56+) were high only in the disease control group. Significant increases in CD4+CD16+ and CD4+CD56+ cell subsets in Behçet's patients and disease controls suggest that T cell activation patterns of these subsets in Behçet's disease are similar to those in other inflammatory disorders.